The Project Gutenberg eBook, Mendelism, by Reginald Crundall Punnett
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Title: Mendelism
Third Edition
Author: Reginald Crundall Punnett
Release Date: May 18, 2009 [eBook #28775]
Language: English
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Transcriber's note:
A few typographical errors have been corrected: they are listed
at the end of the text.
Fig. 8 has been re-mastered to match the text (the Black boxes
were shown as Albino and the heterozygous Albinos as Black).
Superscripted numbers are indicated by a carat character
followed by the superscript. For example, 2^4 denotes 2 raised
to the fourth power and 2^(10) denotes 2 raised to the tenth
power.
Subscripted numbers are indicated by an underscore followed by
the subscript. For example, in the expression "F_1" the 1 is
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[Illustration]
MENDELISM
by
R. C. PUNNETT
Fellow of Gonville and Caius College
Professor of Biology in the University Of Cambridge
THIRD EDITION
Entirely Rewritten and Much Enlarged
New York
The MacMillan Company
1911
All rights reserved
Copyright, 1911,
by The MacMillan Company.
Set up and electrotyped. Published May, 1911.
Norwood Press
J. S. Cushing Co.--Berwick & Smith Co.
Norwood, Mass., U.S.A.
{v}
PREFACE
A few years ago I published a short sketch of Mendel's discovery in
heredity, and of some of the recent experiments which had arisen from it.
Since then progress in these studies has been rapid, and the present
account, though bearing the same title, has been completely rewritten. A
number of illustrations have been added, and here I may acknowledge my
indebtedness to Miss Wheldale for the two coloured plates of sweet peas, to
the Hon. Walter Rothschild for the butterflies figured on Plate VI., to
Professor Wood for photographs of sheep, and to Dr. Drinkwater for the
figures of human hands. To my former publishers also, Messrs. Bowes and
Bowes, I wish to express my thanks for the courtesy with which they
acquiesced in my desire that the present edition should be published
elsewhere.
As the book is intended to appeal to a wide audience, I have not attempted
to give more experimental instances than were necessary to illustrate the
story, nor have I burdened it with bibliographical reference. The reader
who desires further information may be referred to Mr. Bateson's
indispensable Volume on _Mendel's {vi} Principles of Heredity_ (Cambridge,
1909), where a full account of these matters is readily accessible. Neither
have I alluded to recent cytological work in so far as it may bear upon our
problems. Many of the facts connected with the division of the chromosomes
are striking and suggestive, but while so much difference of opinion exists
as to their interpretation they are hardly suited for popular treatment.
In choosing typical examples to illustrate the growth of our ideas it was
natural that I should give the preference to those with which I was most
familiar. For this reason the book is in some measure a record of the work
accomplished by the Cambridge School of Genetics, and it is not unfair to
say that under the leadership of William Bateson the contributions of this
school have been second to none. But it should not be forgotten that
workers in other European countries, and especially in America, have
amassed a large and valuable body of evidence with which it is impossible
to deal in a small volume of this scope.
It is not long since the English language was enriched by two new
words--Eugenics and Genetics--and their similarity of origin has sometimes
led to confusion between them on the part of those who are innocent of
Greek. Genetics is the term applied to the experimental study of heredity
and variation in animals and plants, and the main concern of its students
is the establishing of law and order among the phenomena {vii} there
encountered. Eugenics, on the other hand, deals with the improvement of the
human race under existing conditions of law and sentiment. The Eugenist has
to take into account the religious and social beliefs and prejudices of
mankind. Other issues are involved besides the purely biological one,
though as time goes on it is coming to be more clearly recognised that the
Eugenic ideal is sharply circumscribed by the facts of heredity and
variation, and by the laws which govern the transmission of qualities in
living things. What these facts, what these laws are, in so far as we at
present know them, I have endeavoured to indicate in the following pages;
for I feel convinced that if the Eugenist is to achieve anything solid it
is upon them that he must primarily build. Little enough material, it is
true, exists at present, but that we now see to be largely a question of
time and means. Whatever be the outcome, whatever the form of the structure
which is eventually to emerge, we owe it first of all to Mendel that the
foundations can be well and truly laid.
R. C. P.
CAMBRIDGE, _March, 1911_.
* * * * *
{ix}
CONTENTS
CHAPTER I PAGE
The Problem 1
CHAPTER II
Historical 8
CHAPTER III
Mendel's Work 17
CHAPTER IV
The Presence and Absence Theory 29
CHAPTER V
Interaction of Factors 42
CHAPTER VI
Reversion 59
CHAPTER VII
Dominance 68
{x}
CHAPTER VIII
Wild Forms and Domestic Varieties 79
CHAPTER IX
Repulsion and Coupling of Factors 88
CHAPTER X
Sex 99
CHAPTER XI
Sex (_continued_) 115
CHAPTER XII
Intermediates 125
CHAPTER XIII
Variation and Evolution 135
CHAPTER XIV
Economical 153
CHAPTER XV
Man 170
APPENDIX 187
INDEX 191
* * * * *
{xi}
ILLUSTRATIONS
PLATES
PLATE PAGE
Gregor Mendel _Frontispiece_
I. Rabbits _To face_ 60
II. Sweet Peas " 64
III. Sheep " 78
IV. Sweet Peas " 80
V. Fowls " 107
VI. Butterflies " 146
FIGURES IN TEXT
FIG.
1. Scheme of Inheritance in simple Mendelian Case 21
2. Feathers of Silky and Common Fowl 30
3. Single and Double Primulas 31
4. Fowls' Combs 32
5. Diagram of Inheritance of Fowls' Combs 37
6. Fowls' Combs 39
7. Diagram of F_2 Generation resulting from Cross between
two White Sweet Peas 46
8. Diagram illustrating 9 : 3 : 4 Ratio in Mice 52
9. Sections of Primulas 55
{xii}
10. Small and Large-eyed Primulas 56
11. Diagram illustrating Reversion in Pigeons 67
12. _Primula sinensis_ × _Primula stellata_ 68
13. Diagram illustrating Cross between Dominant and
Recessive White Fowls 72
14. Bearded and Beardless Wheat 75
15. Feet of Fowls 76
16. Scheme of Inheritance of Horns in Sheep 76
17. _Abraxas grossulariata_ and var. _lacticolor_ 99
18. Scheme of Inheritance in _Abraxas_ 102
19. Scheme of Inheritance of Silky Hen × Brown Leghorn
Cock 105
20. Scheme of Inheritance of Brown Leghorn Hen × Silky
Cock 106
21. Scheme of F_1 (ex Brown Leghorn × Silky Cock) crossed
with pure Brown Leghorn 107
22. Scheme for Silky Hen × Brown Leghorn Cock 108
23. Scheme for Brown Leghorn Hen × Silky Cock 109
24. Diagram illustrating Nature of Offspring from Brown Leghorn
Hen × F_1 Cock 110
25. Scheme to illustrate Heterozygous Nature of Brown Leghorn
Hen 111
26. Scheme of Inheritance of Colour-blindness 117
27. Single and Double Stocks 122
28. F_2 Generation ex Silky Hen × Brown Leghorn Cock 127
29. Pedigree of Eurasian Family 130
30. Curve illustrating Influence of Selection 159
{xiii}
31. Curve illustrating Conception of pure Lines 162
32. Brachydactylous and Normal Hands 170
33. Radiograph of Brachydactylous Hand 170
34. Pedigree of Brachydactylous Family 173
35. Pedigree of Hæmophilic Family 175
* * * * *
{xiv}
For although it be a more new and difficult way, to find out the nature
of things, by the things themselves; then by reading of Books, to take
our knowledge upon trust from the opinions of Philosophers: yet must it
needs be confessed, that the former is much more open, and lesse
fraudulent, especially in the Secrets relating to _Natural Philosophy_.
WILLIAM HARVEY,
_Anatomical Exercitations_, 1653.
* * * * *
{1}
CHAPTER I
THE PROBLEM
A curious thing in the history of human thought so far as literature
reveals it to us is the strange lack of interest shown in one of the most
interesting of all human relationships. Few if any of the more primitive
peoples seem to have attempted to define the part played by either parent
in the formation of the offspring, or to have assigned peculiar powers of
transmission to them, even in the vaguest way. For ages man must have been
more or less consciously improving his domesticated races of animals and
plants, yet it is not until the time of Aristotle that we have clear
evidence of any hypothesis to account for these phenomena of heredity. The
production of offspring by man was then held to be similar to the
production of a crop from seed. The seed came from the man, the woman
provided the soil. This remained the generally accepted view for many
centuries, and it was not until the recognition of woman as more than a
passive agent that the physical basis of heredity became established. That
recognition was effected by the microscope, for only with its advent was
actual {2} observation of the minute sexual cells made possible. After more
than a hundred years of conflict lasting until the end of the eighteenth
century, scientific men settled down to the view that each of the sexes
makes a definite material contribution to the offspring produced by their
joint efforts. Among animals the female contributes the ovum and the male
the spermatozoon; among plants the corresponding cells are the ovules and
pollen grains.
As a general rule it may be stated that the reproductive cells produced by
the female are relatively large and without the power of independent
movement. In addition to the actual living substance which is to take part
in the formation of a new individual, the ova are more or less heavily
loaded with the yolk substance that is to provide for the nutrition of the
developing embryo during the early stages of its existence. The size of the
ova varies enormously in different animals. In birds and reptiles where the
contents of the egg form the sole resources of the developing young they
are very large in comparison with the size of the animal which lays them.
In mammals, on the other hand, where the young are parasitic upon the
mother during the earlier stages of their growth, the eggs are minute and
only contain the small amount of yolk that enables them to reach the stage
at which they develop the processes for attaching themselves to the wall of
the maternal uterus. But whatever the differences in the size and
appearance of the ova produced by different {3} animals, they are all
comparable in that each is a distinct and separate sexual cell which, as a
rule, is unable to develop into a new individual of its species unless it
is fertilised by union with a sexual cell produced by the male.
The male sexual cells are always of microscopic size and are produced in
the generative gland or testis in exceedingly large numbers. In addition to
their minuter size they differ from the ova in their power of active
movement. Animals present various mechanisms by which the sexual elements
may be brought into juxtaposition, but in all cases some distance must be
traversed in a fluid or semifluid medium (frequently within the body of the
female parent) before the necessary fusion can occur. To accomplish this
latter end of its journey the spermatozoon is endowed with some form of
motile apparatus, and this frequently takes the form of a long flagellum,
or whip-like process, by the lashing of which the little creature propels
itself much as a tadpole with its tail.
In plants as in animals the female cells or ovules are larger than the
pollen grains, though the disparity in size is not nearly so marked. Still
they are always relatively minute cells since the circumstances of their
development as parasites upon the mother plant render it unnecessary for
them to possess any great supply of food yolk. The ovules are found
surrounded by maternal tissue in the ovary, but through the stigma and down
the pistil a {4} potential passage is left for the male cell. The majority
of flowers are hermaphrodite, and in many cases they are also
self-fertilising. The anthers burst and the contained pollen grains are
then shed upon the stigma. When this happens, the pollen cell slips through
a little hole in its coat and bores its way down the pistil to reach an
ovule in the ovary. Complete fusion occurs, and the minute embryo of a new
plant immediately results. But for some time it is incapable of leading a
separate existence, and, like the embryo mammal, it lives as a parasite
upon its parent. By the parent it is provided with a protective wrapping,
the seed coat, and beneath this the little embryo swells until it reaches a
certain size, when as a ripe seed it severs its connection with the
maternal organism. It is important to realise that the seed of a plant is
not a sexual cell but a young individual which, except for the coat that it
wears, belongs entirely to the next generation. It is with annual plants in
some respects as with many butterflies. During one summer they are
initiated by the union of two sexual cells and pass through certain stages
of larval development--the butterfly as a caterpillar, the plant as a
parasite upon its mother. As the summer draws to a close each passes into a
resting-stage against the winter cold--the butterfly as a pupa and the
plant as a seed, with the difference that while the caterpillar provides
its own coat, that of the plant is provided by its mother. With the advent
of spring both butterfly and {5} plant emerge, become mature, and
themselves ripen germ cells which give rise to a new generation.
Whatever the details of development, one cardinal fact is clear. Except for
the relatively rare instances of parthenogenesis a new individual, whether
plant or animal, arises as the joint product of two sexual cells derived
from individuals of different sexes. Such sexual cells, whether ovules or
ova, spermatozoa or pollen grains, are known by the general term of
GAMETES, or marrying cells, and the individual formed by the fusion or
yoking together of two gametes is spoken of as a ZYGOTE. Since a zygote
arises from the yoking together of two separate gametes, the individual so
formed must be regarded throughout its life as a double structure in which
the components brought in by each of the gametes remain intimately fused in
a form of partnership. But when the zygote in its turn comes to form
gametes, the partnership is broken and the process is reversed. The
component parts of the dual structure are resolved, with the formation of a
set of single structures, the gametes.
The life cycle of a species from among the higher plants or animals may be
regarded as falling into three periods: (1) a period of isolation in the
form of gametes, each a living unit incapable of further development
without intimate association with another produced by the opposite sex; (2)
a period of association in which two gametes become yoked together into a
zygote and react upon one {6} another to give rise by a process of cell
division to what we ordinarily term an individual with all its various
attributes and properties; and (3) a period of dissociation when the single
structured gametes separate out from that portion of the double structured
zygote which constitutes its generative gland. What is the relation between
gamete and zygote, between zygote and gamete? how are the properties of the
zygote represented in the gamete, and in what manner are they distributed
from the one to the other?--these are questions which serve to indicate the
nature of the problem underlying the process of heredity.
Owing to their peculiar power of growth and the relatively large size to
which they attain, many of the properties of zygotes are appreciable by
observation. The colour of an animal or of a flower, the shape of a seed,
or the pattern on the wings of a moth are all zygotic properties, and all
capable of direct estimation. It is otherwise with the properties of
gametes. While the difference between a black and a white fowl is
sufficiently obvious, no one by inspection can tell the difference between
the egg that will hatch into a black and that which will hatch into a
white. Nor from a mass of pollen grains can any one to-day pick out those
that will produce white from those that will produce coloured flowers.
Nevertheless, we know that in spite of apparent similarity there must exist
fundamental differences among the gametes, even {7} among those that spring
from the same individual. At present our only way of appreciating those
differences is to observe the properties of the zygotes which they form.
And as it takes two gametes to form a zygote, we are in the position of
attempting to decide the properties of two unknowns from one known.
Fortunately the problem is not entirely one of simple mathematics. It can
be attacked by the experimental method, and with what measure of success
will appear in the following pages.
* * * * *
{8}
CHAPTER II
HISTORICAL
To Gregor Mendel, monk and abbot, belongs the credit of founding the modern
science of heredity. Through him there was brought into these problems an
entirely new idea, an entirely fresh conception of the nature of living
things. Born in 1822 of Austro-Silesian parentage, he early entered the
monastery of Brünn, and there in the seclusion of the cloister garden he
carried out with the common pea the series of experiments which has since
become so famous. In 1865 after eight years' work he published the results
of his experiments in the _Proceedings of the Natural History Society of
Brünn_, in a brief paper of some forty pages. But brief as it is the
importance of the results and the lucidity of the exposition will always
give it high rank among the classics of biological literature. For
thirty-five years Mendel's paper remained unknown, and it was not until
1900 that it was simultaneously discovered by several distinguished
botanists. The causes of this curious neglect are not altogether without
interest. Hybridisation experiments before Mendel there had been in plenty.
The classificatory work of {9} Linnaeus in the latter half of the
eighteenth century had given a definite significance to the word species,
and scientific men began to turn their attention to attempting to discover
how species were related to one another. And one obvious way of attacking
the problem was to cross different species together and see what happened.
This was largely done during the earlier half of the nineteenth century,
though such work was almost entirely confined to the botanists. Apart from
the fact that plants lend themselves to hybridisation work more readily
than animals, there was probably another reason why zoologists neglected
this form of investigation. The field of zoology is a wider one than that
of botany, presenting a far greater variety of type and structure. Partly
owing to their importance in the study of medicine, and partly owing to
their smaller numbers, the anatomy of the vegetable was far better known
than that of the animal kingdom. It is, therefore, not surprising that the
earlier part of the nineteenth century found the zoologists, under the
influence of Cuvier and his pupils, devoting their entire energies to
describing the anatomy of the new forms of animal life which careful search
at home and fresh voyages of discovery abroad were continually bringing to
light. During this period the zoologist had little inclination or
inducement to carry on those investigations in hybridisation which were
occupying the attention of some botanists. Nor did the efforts of the
botanists afford much {10} encouragement to such work, for in spite of the
labour devoted to these experiments, the results offered but a confused
tangle of facts, contributing in no apparent way to the solution of the
problem for which they had been undertaken. After half a century of
experimental hybridisation the determination of the relation of species and
varieties to one another seemed as remote as ever. Then in 1859 came the
_Origin of Species_, in which Darwin presented to the world a consistent
theory to account for the manner in which one species might have arisen
from another by a process of gradual evolution. Briefly put, that theory
was as follows: In any species of plant or animal the reproductive capacity
tends to outrun the available food supply, and the resulting competition
leads to an inevitable struggle for existence. Of all the individuals born,
only a portion, and that often a very small one, can survive to produce
offspring. According to Darwin's theory, the nature of the surviving
portion is not determined by chance alone. No two individuals of a species
are precisely alike, and among the variations that occur some enable their
possessors to cope more successfully with the competitive conditions under
which they exist. In comparison with their less favoured brethren they have
a better chance of surviving in the struggle for existence and consequently
of leaving offspring. The argument is completed by the further assumption
of a principle of heredity, in virtue of which offspring tend to {11}
resemble their parents more than other members of the species. Parents
possessing a favourable variation tend to transmit that variation to their
offspring, to some in greater, to others in less degree. Those possessing
it in greater degree will again have a better chance of survival, and will
transmit the favourable variation in even greater degree to some of their
offspring. A competitive struggle for existence working in combination with
certain principles of variation and heredity results in a slow and
continuous transformation of species through the operation of a process
which Darwin termed natural selection.
The coherence and simplicity of the theory, supported as it was by the
great array of facts which Darwin had patiently marshalled together,
rapidly gained the enthusiastic support of the great majority of
biologists. The problem of the relation of species at last appeared to be
solved, and for the next forty years zoologists and botanists were busily
engaged in classifying by the light of Darwin's theory the great masses of
anatomical facts which had already accumulated and in adding and
classifying fresh ones. The study of comparative anatomy and embryology
received a new stimulus, for with the acceptance of the theory of descent
with modification it became incumbent upon the biologist to demonstrate the
manner in which animals and plants differing widely in structure and
appearance could be conceivably related to one another. Thenceforward the
energies of both {12} botanists and zoologists have been devoted to the
construction of hypothetical pedigrees suggesting the various tracks of
evolution by which one group of animals or plants may have arisen from
another through a long continued process of natural selection. The result
of such work on the whole may be said to have shown that the diverse forms
under which living things exist to-day, and have existed in the past so far
as palaeontology can tell us, are consistent with the view that they are
all related by the community of descent which the accepted theory of
evolution demands, though as to the exact course of descent for any
particular group of animals there is often considerable diversity of
opinion. It is obvious that all this work has little or nothing to do with
the manner in which species are formed. Indeed, the effect of Darwin's
_Origin of Species_ was to divert attention from the way in which species
originate. At the time that it was put forward his explanation appeared so
satisfying that biologists accepted the notions of variation and heredity
there set forth and ceased to take any further interest in the work of the
hybridisers. Had Mendel's paper appeared a dozen years earlier it is
difficult to believe that it could have failed to attract the attention it
deserved. Coming as it did a few years after the publication of Darwin's
great work, it found men's minds set at rest on the problems that he raised
and their thoughts and energies directed to other matters. {13}
Nevertheless one interesting and noteworthy attempt to give greater
precision to the term heredity was made about this time. Francis Galton, a
cousin of Darwin, working upon data relating to the breeding of Basset
hounds, found that he could express on a definite statistical scheme the
proportion in which the different colours appeared in successive
generations. Every individual was conceived of as possessing a definite
heritage which might be expressed as unity. Of this, ½ was on the average
derived from the two parents (_i.e._ ¼ from each parent), ¼ from the four
grandparents, 1/8 from the eight great-grandparents, and so on. _The Law of
Ancestral Heredity_, as it was termed, expresses with fair accuracy some of
the statistical phenomena relating to the transmission of characters in a
mixed population. But the problem of the way in which characters are
distributed from gamete to zygote and from zygote to gamete remained as
before. Heredity is essentially a physiological problem, and though
statistics may be suggestive in the initiation of experiment, it is upon
the basis of experimental fact that progress must ultimately rest. For this
reason, in spite of its ingenuity and originality, Galton's theory and the
subsequent statistical work that has been founded upon it failed to give us
any deeper insight into the nature of the hereditary process.
While Galton was working in England the German zoologist August Weismann
was elaborating the complicated {14} theory of heredity which eventually
appeared in his work on _The Germplasm_ (1885), a book which will be
remembered for one notable contribution to the subject. Until the
publication of Weismann's work it had been generally accepted that the
modifications brought about in the individual during its lifetime, through
the varying conditions of nutrition and environment, could be transmitted
to the offspring. In this biologists were but following Darwin, who held
that the changes in the parent resulting from increased use or disuse of
any part or organ were passed on to the children. Weismann's theory
involved the conception of a sharp cleavage between the general body
tissues or somatoplasm and the reproductive glands or germplasm. The
individual was merely a carrier for the essential germplasm whose
properties had been determined long before he was capable of leading a
separate existence. As this conception ran counter to the possibility of
the inheritance of "acquired characters," Weismann challenged the evidence
upon which it rested and showed that it broke down wherever it was
critically examined. By thus compelling biologists to revise their ideas as
to the inherited effects of use and disuse, Weismann rendered a valuable
service to the study of genetics and did much to clear the way for
subsequent research.
A further important step was taken in 1895, when Bateson once more drew
attention to the problem of the origin {15} of species, and questioned
whether the accepted ideas of variation and heredity were after all in
consonance with the facts. Speaking generally, species do not grade
gradually from one to the other, but the differences between them are sharp
and specific. Whence comes this prevalence of discontinuity if the process
by which they have arisen is one of accumulation of minute and almost
imperceptible differences? Why are not intermediates of all sorts more
abundantly produced in nature than is actually known to be the case?
Bateson saw that if we are ever to answer this question we must have more
definite knowledge of the nature of variation and of the nature of the
hereditary process by which these variations are transmitted. And the best
way to obtain that knowledge was to let the dead alone and to return to the
study of the living. It was true that the past record of experimental
breeding had been mainly one of disappointment. It was true also that there
was no tangible clue by which experiments might be directed in the present.
Nevertheless in this kind of work alone there seemed any promise of
ultimate success.
A few years later appeared the first volume of de Vries' remarkable book on
_The Mutation Theory_. From a prolonged study of the evening primrose
(_Oenothera_) de Vries concluded that new varieties suddenly arose from
older ones by sudden sharp steps or mutations, and not by any process
involving the gradual accumulation of minute {16} differences. The number
of striking cases from among widely different plants which he was able to
bring forward went far to convincing biologists that discontinuity in
variation was a more widespread phenomenon than had hitherto been
suspected, and not a few began to question whether the account of the mode
of evolution so generally accepted for forty years was after all the true
account. Such in brief was the outlook in the central problem of biology at
the time of the rediscovery of Mendel's work.
* * * * *
{17}
CHAPTER III
MENDEL'S WORK
The task that Mendel set before himself was to gain some clear conception
of the manner in which the definite and fixed varieties found within a
species are related to one another, and he realised at the outset that the
best chance of success lay in working with material of such a nature as to
reduce the problem to its simplest terms. He decided that the plant with
which he was to work must be normally self-fertilising and unlikely to be
crossed through the interference of insects, while at the same time it must
possess definite fixed varieties which bred true to type. In the common pea
(_Pisum sativum_) he found the plant he sought. A hardy annual, prolific,
easily worked, _Pisum_ has a further advantage in that the insects which
normally visit flowers are unable to gather pollen from it and so to bring
about cross fertilisation. At the same time it exists in a number of
strains presenting well-marked and fixed differences. The flowers may be
purple, or red, or white; the plants may be tall or dwarf; the ripe seeds
may be yellow or green, round or wrinkled--such are a few of the characters
in which the various races of peas differ from one another. {18}
In planning his crossing experiments Mendel adopted an attitude which
marked him off sharply from the earlier hybridisers. He realised that their
failure to elucidate any general principle of heredity from the results of
cross fertilisation was due to their not having concentrated upon
particular characters or traced them carefully through a sequence of
generations. That source of failure he was careful to avoid, and throughout
his experiments he crossed plants presenting sharply contrasted characters,
and devoted his efforts to observing the behaviour of these characters in
successive generations. Thus in one series of experiments he concentrated
his attention on the transmission of the characters tallness and dwarfness,
neglecting in so far as these experiments were concerned any other
characters in which the parent plants might differ from one another. For
this purpose he chose two strains of peas, one of about 6 feet in height,
and another of about 1½ feet. Previous testing had shown that each strain
bred true to its peculiar height. These two strains were artificially
crossed[1] with one another, and it was found to make no difference which
was used as the pollen parent and which was used as the ovule parent. In
either case the result was the same. The result of crossing tall with dwarf
was in every case nothing but talls, as tall or even a little taller than
the tall parent. For this reason Mendel termed tallness the DOMINANT and
{19} dwarfness the RECESSIVE character. The next stage was to collect and
sow the seeds of these tall hybrids. Such seeds in the following year gave
rise to a mixed generation consisting of talls and dwarfs _but no
intermediates_. By raising a considerable number of such plants Mendel was
able to establish the fact that the number of talls which occurred in this
generation was almost exactly three times as great as the number of the
dwarfs. As in the previous year, seed were carefully collected from this,
the second hybrid generation, and in every case _the seeds from each
individual plant were harvested separately and separately sown in the
following year_. By this respect for the individuality of the different
plants, however closely they resembled one another, Mendel found the clue
that had eluded the efforts of all his predecessors. The seeds collected
from the dwarf recessives bred true, giving nothing but dwarfs. And this
was true for every dwarf tested. But with the talls it was quite otherwise.
Although indistinguishable in appearance, some of them bred true, while
others behaved like the original tall hybrids, giving a generation
consisting of talls and dwarfs in the proportion of three of {20} the
former to one of the latter. Counting showed that the number of the talls
which gave dwarfs was double that of the talls which bred true.
T × D --------------------- P
|
T(D) --------------------- F_1
|
+-----------+-----------+-------------+
T T(D) T(D) D --- F_2
| +---+----+--+ +--+----+---+ |
T T T(D) T(D) D T T(D) T(D) D D --- F_3
| |
T D --- F_4
If we denote a dwarf plant as D, a true breeding tall plant as T, and a
tall which gives both talls and dwarfs in the ratio 3 : 1 as T(D), the
result of these experiments may be briefly summarised in the foregoing
scheme.[2]
Mendel experimented with other pairs of contrasted characters and found
that in every instance they followed the same scheme of inheritance. Thus
coloured flowers were dominant to white, in the ripe seeds yellow was
dominant to green, and round shape was dominant to wrinkled, and so on. In
every case where the inheritance of an alternative pair of characters was
concerned the effect of the cross in successive generations was to produce
three and only three different sorts of individuals, viz. dominants which
bred true, dominants which gave both dominant and recessive offspring in
the ratio 3 : 1, and recessives which always bred true. Having determined a
general scheme of inheritance which experiment showed to hold good for each
of the seven pairs of alternative characters with which he worked, Mendel
set himself to providing a theoretical interpretation of this scheme which,
as he clearly realised, must be in terms of germ cells. He {21} conceived
of the gametes as bearers of something capable of giving rise to the
characters of the plant, but he regarded any individual gamete as being
able to carry one and one only of any alternative pair of characters. A
given gamete could carry tallness _or_ dwarfness, but not both. The two
were mutually exclusive so far as the gamete was concerned. It must be pure
for one or the other of such a pair, and this conception of the purity of
the gametes is the most essential part of Mendel's theory.
[Illustration: FIG. 1.
Scheme of inheritance in the cross of tall with dwarf pea. Gametes
represented by small and zygotes by larger circles.]
We may now proceed with the help of the accompanying scheme (Fig. 1) to
deduce the results that should flow from Mendel's conception of the nature
of the gametes, and to see how far they are in accordance with the facts.
Since the original tall plant belonged to a strain which bred true, all the
gametes produced by it must bear the tall character. Similarly all the
gametes of the original dwarf plant must bear the dwarf character. A cross
between these two means the union of {22} a gamete containing tallness with
one bearing dwarfness. Owing to the completely dominant nature of the tall
character, such a plant is in appearance indistinguishable from the pure
tall, but it differs markedly from it in the nature of the gametes to which
it gives rise. When the formation of the gametes occurs, the elements
representing dwarfness and tallness SEGREGATE from one another, so that
half of the gametes produced contain the one, and half contain the other of
these two elements. For on hypothesis every gamete must be pure for one or
other of these two characters. And this is true for the ovules as well as
for the pollen grains. Such hybrid F_1 plants, therefore, must produce a
series of ovules consisting of those bearing tallness and those bearing
dwarfness, and must produce them in equal numbers. And similarly for the
pollen grains. We may now calculate what should happen when such a series
of pollen grains meets such a series of ovules, _i.e._ the nature of the
generation that should be produced when the hybrid is allowed to fertilise
itself. Let us suppose that there are 4x ovules so that 2x are "tall" and
2x are "dwarf." These are brought in contact with a mass of pollen grains
of which half are "tall" and half are "dwarf." It is obvious that a "tall"
ovule has an equal chance of being fertilised by a "tall" or a "dwarf"
pollen grain. Hence of our 2x "tall" ovules, x will be fertilised by "tall"
pollen grains and x will be fertilised by "dwarf" pollen grains. The former
must give rise to tall {23} plants, and since the dwarf character has been
entirely eliminated from them they must in the future breed true. The
latter must also give rise to tall plants, but since they carry also the
recessive dwarf character they must when bred from produce both tails and
dwarfs. Each of the 2x dwarf ovules, again, has an equal chance of being
fertilised by a "tall" or by a "dwarf" pollen grain. Hence x will give rise
to tall plants carrying the recessive dwarf character, while x will produce
plants from which the tall character has been eliminated, _i.e._ to pure
recessive dwarfs. Consequently from the 4x ovules of the self-fertilised
hybrid we ought to obtain 3x tall and x dwarf plants. And of the 3x talls x
should breed true to tallness, while the remaining 2x, having been formed
like the original hybrid by the union of a "tall" and a "dwarf" gamete,
ought to behave like it when bred from and give talls and dwarfs in the
ratio 3 : 1. Now this is precisely the result actually obtained by
experiment (cf. p. 17), and the close accord of the experimental results
with those deduced on the assumption of the purity of the gametes as
enunciated by Mendel affords the strongest of arguments for regarding the
nature of the gametes and their relation to the characters of the zygotes
in the way that he has done.
It is possible to put the theory to a further test. The explanation of the
3 : 1 ratio of dominants and recessives in the F_2 generation is regarded
as due to the F_1 individuals producing equal numbers of gametes bearing
the {24} dominant and recessive elements respectively. If now the F_1 plant
be crossed with the pure recessive, we are bringing together a series of
gametes consisting of equal numbers of dominants and recessives with a
series consisting solely of recessives. We ought from such a cross to
obtain equal numbers of dominant and recessive individuals, and further,
the dominants so produced ought all to give both dominants and recessives
in the ratio 3 : 1 when they themselves are bred from. Both of these
expectations were amply confirmed by experiment, and crossing with the
recessive is now a recognised way of testing whether a plant or animal
bearing a dominant character is a pure dominant, or an impure dominant
which is carrying the recessive character. In the former case the offspring
will be all of the dominant form, while in the latter they will consist on
the average of equal numbers of dominants and recessives.
So far we have been concerned with the results obtained when two
individuals differing in a single pair of characters are crossed together
and with the interpretation of those results. But Mendel also used plants
which differed in more than a single pair of differentiating characters. In
such cases he found that each pair of characters followed the same definite
rule, but that the inheritance of each pair was absolutely independent of
the other. Thus, for example, when a tall plant bearing coloured flowers
was crossed with a dwarf plant {25} bearing white flowers the resulting
hybrid was a tall plant with coloured flowers. For coloured flowers are
dominant to white, and tallness is dominant to dwarfness. In the succeeding
generation there are plants with coloured flowers and plants with white
flowers in the proportion of 3 : 1, and at the same time tall plants and
dwarf plants in the same proportion. Hence the chances that a tall plant
will have coloured flowers are three times as great as its chance of having
white flowers. And this is also true for the dwarf plants. As the result of
this cross, therefore, we should expect an F_2 generation consisting of
four classes, viz. coloured talls, white talls, coloured dwarfs, and white
dwarfs, and we should further expect these four forms to appear in the
ratio of 9 coloured talls, 3 white talls, 3 coloured dwarfs, and 1 white
dwarf. For this is the only ratio which satisfies the conditions that the
talls should be to the dwarfs as 3 : 1, and at the same time the coloured
should be to the whites as 3 : 1. And these are the proportions that Mendel
found to obtain actually in his experiments. Put in a more general form, it
may be stated that when two individuals are crossed which differ in two
pairs of differentiating characters the hybrids (F_1) are all of the same
form, exhibiting the dominant character of each of the two pairs, while the
F_2 generation produced by such hybrids consists on the average of 9
showing both dominants, 3 showing one dominant and one recessive, {26} 3
showing the other dominant and the other recessive, and 1 showing both
recessive characters. And, as Mendel pointed out, the principle may be
extended indefinitely. If, for example, the parents differ in three pair of
characters A, B, and C, respectively dominant to a, b, and c, the F_1
individuals will be all of the form ABC, while the F_2 generation will
consists of 27 ABC, 9 ABc, 9 AbC, 9 aBC, 3 Abc, 3 aBc, 3 abC, and 1 abc.
When individuals differing in a number of alternative characters are
crossed together, the hybrid generation, provided that the original parents
were of pure strains, consists of plants of the same form; but when these
are bred from a redistribution of the various characters occurs. That
redistribution follows the same definite rule for each character, and if
the constitution of the original parents be known, the nature of the F_2
generation, _i.e._ the number of possible forms and the proportions in
which they occur, can be readily calculated. Moreover, as Mendel showed, we
can calculate also the chances of any given form breeding true. To this
point, however, we shall return later.
Of Mendel's experiments with beans it is sufficient to say here that they
corroborated his more ample work with peas. He is also known to have made
experiments with many other plants, and a few of his results are
incidentally given in his series of letters to Nägeli the botanist. To the
breeding and crossing of bees he also devoted much {27} time and attention,
but unhappily the record of these experiments appears to have been lost.
The only other published work that we possess dealing with heredity is a
brief paper on some crossing experiments with the hawkweeds (_Hieracium_),
a genus that he chose for working with because of the enormous number of
forms under which it naturally exists. By crossing together the more
distinct varieties, he evidently hoped to produce some of these numerous
wild forms, and so throw light upon their origin and nature. In this hope
he was disappointed. Owing in part to the great technical difficulties
attending the cross fertilisation of these flowers he succeeded in
obtaining very few hybrids. Moreover, the behaviour of those which he did
obtain was quite contrary to what he had found in the peas. Instead of
giving a variety of forms in the F_2 generation, they bred true and
continued to do so as long as they were kept under observation. More recent
research has shown that this is due to a peculiar form of parthenogenesis
(cf. p. 135), and not to any failure of the characters to separate clearly
from one another in the gametes. Mendel, however, could not have known of
this, and his inability to discover in _Hieracium_ any indication of the
rule which he had found to hold good for both peas and beans must have been
a source of considerable disappointment. Whether for this reason, or owing
to the utter neglect of his work by the scientific world, Mendel gave up
his experimental {28} researches during the latter part of his life. His
closing years were shadowed with ill-health and embittered by a controversy
with the Government on a question of the rights of his monastery. He died
of Bright's disease in 1884.
_Note._--Shortly after the discovery of Mendel's paper a need was felt
for terms of a general nature to express the constitution of
individuals in respect of inherited characters, and Bateson accordingly
proposed the words homozygote and heterozygote. An individual is said
to be homozygous for a given character when it has been formed by two
gametes each bearing the character, and all the gametes of a homozygote
bear the character in respect of which it is homozygous. When, however,
the zygote is formed by two gametes of which one bears the given
character while the other does not, it is said to be heterozygous for
the character in question, and only half the gametes produced by such a
heterozygote bear the character. An individual may be homozygous for
one or more characters, and at the same time may be heterozygous for
others.
* * * * *
{29}
CHAPTER IV
THE PRESENCE AND ABSENCE THEORY
It was fortunate for the development of biological science that the
rediscovery of Mendel's work found a small group of biologists deeply
interested in the problems of heredity, and themselves engaged in
experimental breeding. To these men the extraordinary significance of the
discovery was at once apparent. From their experiments, undertaken in
ignorance of Mendel's paper, de Vries, Correns, and Tschermak were able to
confirm his results in peas and other plants, while Bateson was the first
to demonstrate their application to animals. Thenceforward the record has
been one of steady progress, and the result of ten years' work has been to
establish more and more firmly the fundamental nature of Mendel's
discovery. The scheme of inheritance, which he was the first to enunciate,
has been found to hold good for such diverse things as height, hairiness,
and flower colour and flower form in plants, the shape of pollen grains,
and the structure of fruits; while among animals the coat colour of
mammals, the form of the feathers and of the comb in poultry, the waltzing
habit of Japanese mice, and eye {30} colour in man are but a few examples
of the diversity of characters which all follow the same law of
transmission. And as time went on many cases which at first seemed to fall
without the scheme have been gradually brought into line in the light of
fuller knowledge. Some of these will be dealt with in the succeeding
chapters of this book. Meanwhile we may concern ourselves with the single
modification of Mendel's original views which has arisen out of more ample
knowledge.
[Illustration: FIG. 2.
A wing feather and a contour feather of an ordinary and a silky fowl. The
peculiar ragged appearance of the silky feathers is due to the absence of
the little hooks or barbules which hold the barbs together. The silky
condition is recessive.]
As we have already seen, Mendel considered that in the gamete there was
either a definite something {31} corresponding to the dominant character or
a definite something corresponding to the recessive character, and that
these somethings whatever they were could not coexist in any single gamete.
For these somethings we shall in future use the term FACTOR. The factor,
then, is what corresponds in the gamete to the UNIT-CHARACTER that appears
in some shape or other in the development of the zygote. Tallness in the
pea is a unit-character, and the gametes in which it is represented are
said to contain the factor for tallness. Beyond their existence in the
gamete and their mode of transmission we make no suggestion as to the
nature of these factors.
[Illustration: FIG. 3.
Two double and an ordinary single primula flower. This form of double is
recessive to the single.]
{32}
[Illustration: FIG. 4.
Fowls' combs. A, pea; B, rose; C, single; D, walnut.]
On Mendel's view there was a factor corresponding to the dominant character
and another factor corresponding to the recessive character of each
alternative pair of unit-characters, and the characters were alternative
because no gamete could carry more than one of the two factors belonging to
the alternative pair. On the other hand, Mendel supposed that it always
carried either one or the other of such a pair. As experimental work
proceeded, {33} it soon became clear that there were cases which could not
be expressed in terms of this conception. The nature of the difficulty and
the way in which it was met will perhaps be best understood by considering
a set of experiments in which it occurred. Many of the different breeds of
poultry are characterised by a particular form of comb, and in certain
cases the inheritance of these has been carefully worked out. It was shown
that the rose comb (Fig. 4, B) with its flattened papillated upper surface
and backwardly projecting pike was dominant in the ordinary way to the
deeply serrated high single comb (Fig. 4, C) which is characteristic of the
Mediterranean races. Experiment also showed that the pea comb (Fig. 4, A),
a form with a low central and two well-developed lateral ridges, such as is
found in Indian game, behaves as a simple dominant to the single comb. The
interesting question arose as to what would happen when the rose and the
pea, two forms each dominant to the same third form, were mated together.
It seemed reasonable to suppose that things which were alternative to the
same thing would be alternative to one another--that either rose or pea
would dominate in the hybrids, and that the F_2 generation would consist of
dominants and recessives in the ratio 3 : 1. The result of the experiment
was, however, very different. The cross rose × pea led to the production of
a comb quite unlike either of them. This, the so-called walnut comb (Fig.
4, D), {34} from its resemblance to the half of a walnut, is a type of comb
which is normally characteristic of the Malay fowl. Moreover, when these
F_1 birds were bred together, a further unlooked-for result was obtained.
As was expected, there appeared in the F_2 generation the three forms
walnut, rose, and pea. But there also appeared a definite proportion of
single-combed birds, and among many hundreds of chickens bred in this way
the proportions in which the four forms walnut, rose, pea, and single
appeared was 9 : 3 : 3 : 1.
Rose × Pea
|
+----+----+
Walnut × Walnut
|
+--------+---+---+--------+
Walnut Rose Pea Single
(9) (3) (3) (1)
Now this, as Mendel showed, is the ratio found in an F_2 generation when
the original parents differ in two pairs of alternative characters, and
from the proportions in which the different forms of comb occur we must
infer that the walnut contains both dominants, the rose and the pea one
dominant each, while the single is pure for both recessive characters. This
accorded with subsequent breeding experiments, for the singles bred
perfectly true as soon as they had once made their appearance. So far the
case is clear. The difficulty comes when we attempt to define these two
pairs of characters. How are we to express the fact that while single
behaves as a simple recessive to either pure rose, or to pure pea, it can
yet appear in F_2 from a cross {35} between these two pure forms, though
neither of them should, on Mendel's view, contain the single? An
explanation which covers the facts in a simple way is that which has been
termed the "Presence and Absence" theory. On this theory the dominant
character of an alternative pair owes its dominance to the presence of a
factor which is absent in the recessive. The tall pea is tall owing to the
presence in it of the factor for tallness, but in the absence of this
factor the pea remains a dwarf. All peas are dwarf, but the tall is a dwarf
plus a factor which turns it into a tall. Instead of the characters of an
alternative pair being due to two separate factors, we now regard them as
the expression of the only two possible states of a single factor, viz. its
presence or its absence. The conception will probably become clearer if we
follow its application in detail to the case of the fowl's combs. In this
case we are concerned with the transmission of the two factors, rose (R)
and pea (P), the presence of each of which is alternative to its absence.
The rose-combed bird contains the factor for rose but not that for pea, and
the pea-combed bird contains the factor for pea but not that for rose. When
both factors are present in a bird, as in the hybrid made by crossing rose
with pea, the result is a walnut. For convenience of argument we may denote
the presence of a given factor by a capital letter and its absence by the
corresponding small letter. The use of the small letter is merely a
symbolic way of intimating {36} that a particular factor is absent in a
gamete or zygote. Represented thus the zygotic constitution of a pure
rose-combed bird is RRpp; for it has been formed by the union of two
gametes both of which contained R but not P. Similarly we may denote the
pure pea-combed bird as rrPP. On crossing the rose with the pea union
occurs between a gamete Rp and a gamete rP, resulting in the formation of a
heterozygote of the constitution RrPp. The use of the small letters here
informs us that such a zygote contains only a single dose of each of the
factors R and P, although, of course, it is possible for a zygote, if made
in a suitable way, to have a double dose of any factor. Now when such a
bird comes to form gametes a separation takes place between the part of the
zygotic cell containing R and the part which does not contain it (r). Half
of its gametes, therefore, will contain R and the other half will be
without it (r). Similarly half of its gametes will contain P and the other
half will be without it (p). It is obvious that the chances of R being
distributed to a gamete with or without P are equal. Hence the gametes
containing R will be of two sorts, RP and Rp, and these will be produced in
equal numbers. Similarly the gametes without R will also be of two sorts,
rP and rp, and these, again, will be produced in equal numbers. Each of the
hybrid walnut-combed birds, therefore, gives rise to a series consisting of
equal numbers of gametes of the four different types RP, Rp, rP, and rp;
and the breeding {37} together of such F_1 birds means the bringing
together of two such series of gametes. When this happens an ovum of any
one of the four types has an equal chance of being fertilised by a
spermatozoon of any one of the four types. A convenient and simple method
of demonstrating what happens under such circumstances is the method
sometimes termed the "chessboard" method. For two series each consisting of
four different types of gamete we require a square divided up into 16
parts. The four terms of the gametic series are first written horizontally
across the four sets of four squares, so that the series is repeated four
times. It is then written vertically four times, care being taken to keep
to the same order. In this simple mechanical way all the possible
combinations are represented and in their proper proportions.
+-------+-------+-------+-------+
|RP |RP |RP |RP |
|RP |Rp |rP |rp |
| | | | |
| Walnut| Walnut| Walnut| Walnut|
+-------+-------+-------+-------+
|Rp |Rp |Rp |Rp |
|RP |Rp |rP |rp |
| | | | |
| Walnut| Rose| Walnut| Rose|
+-------+-------+-------+-------+
|rP |rP |rP |rP |
|RP |Rp |rP |rp |
| | | | |
| Walnut| Walnut| Pea| Pea|
+-------+-------+-------+-------+
|rp |rp |rp |rp |
|RP |Rp |rP |rp |
| | | | |
| Walnut| Rose| Pea| Single|
+-------+-------+-------+-------+
FIG. 5.
Diagram to illustrate the nature of the F_2 generation from the cross
of rose comb × pea comb.
Fig. 5 shows the result of applying this method to our series RP, Rp, rP,
rp, and the 16 squares represent the different kinds of zygotes formed and
the proportions in which they occur. As {38} the figure shows, 9 zygotes
contain both R and P, having a double or a single dose of either or both of
these factors. Such birds must be all walnut combed. Three out of the 16
zygotes contain R but not P, and these must be rose-combed birds. Three,
again, contain P but not R and must be pea-combed birds. Finally one out of
the 16 contains neither R nor P. It cannot be rose--it cannot be pea. It
must, therefore, be something else. As a matter of fact it is single. Why
it should be single and not something else follows from what we already
know about the behaviour of these various forms of comb. For rose is
dominant to single; therefore on the Presence and Absence theory a rose is
a single plus a factor which turns the single into a rose. If we could
remove the "rose" factor from a rose-combed bird the underlying single
would come into view. Similarly a pea comb is a single plus a factor which
turns the single into a pea, and a walnut is a single which possesses two
additional modifying factors. Singleness, in fact, underlies all these
combs, and if we write their zygotic constitution in full we must denote a
walnut as RRPPSS, a rose as RRppSS, a pea as rrPPSS, and a single as
rrppSS. The crossing of rose with pea results in a reshuffling of the
factors concerned, and in accordance with the principle of segregation some
zygotes are formed in which neither of the modifying factors R and P are
present, and the single character can then become manifest. {39}
The Presence and Absence theory is to-day generally accepted by students of
these matters. Not only does it afford a simple explanation of the
remarkable fact that in all cases of Mendelian inheritance we should be
able to express our unit-characters in terms of alternative pairs, but, as
we shall have occasion to refer to later, it suggests a clue as to the
course by which the various domesticated varieties of plants and animals
have arisen from their wild prototypes.
[Illustration: FIG. 6.
Fowls' combs. A and B, F_1 hen from rose × Breda; C, an F_1 cock from the
cross of single × Breda; D, head of Breda cock.]
Before leaving this topic we may draw attention to some experiments which
offer a pretty confirmation of the view that the rose comb is a single to
which a modifying factor for roseness has been added. It was argued that if
we could find a type of comb in which the factor for singleness was absent,
then on crossing such a comb with a rose we ought, if singleness really
underlies rose, to obtain some single combs in F_2 from such a cross. Such
a comb we had the good fortune to find in the Breda fowl, a breed largely
used in Holland. This fowl is usually spoken of as combless, for the place
of the comb is taken by a covering of short bristlelike feathers (Fig. 6,
D). In reality it possesses the vestige of a comb in the form of two minute
lateral knobs of comb tissue. Characteristic also of this breed is the high
development of the horny nostrils, a feature probably correlated with the
almost complete absence of comb. The first step in the experiment was to
prove the absence of the factor for singleness in the Breda. {40} On
crossing Breda with single the F_1 birds exhibit a large comb of the form
of a double single comb in which the two portions are united anteriorly,
but diverge from one another towards the back of the head (Fig. 6, C). The
Breda contains an element of duplicity which is dominant to the simplicity
of the ordinary single comb. But it cannot contain the factor for the
single comb, because as soon as that is put into it by crossing with a
single the comb {41} assumes a large size, and is totally distinct in
appearance from its almost complete absence in the pure Breda. Now when the
Breda is crossed with the rose duplicity is dominant to simplicity, and
rose is dominant to lack of comb, and the F_1 generation consists of birds
possessing duplex rose combs (Fig. 6, A and B). On breeding such birds
together we obtain a generation consisting of Bredas, duplex roses, roses,
duplex singles, and singles. From our previous experiment we know that the
singles could not have come from the Breda, since a Breda comb to which the
factor for single has been added no longer remains a Breda. Therefore it
must have come from the rose, thus confirming our view that the rose is in
reality a single comb which contains in addition a dominant modifying
factor (R) whose presence turns it into a rose. We shall take it,
therefore, that there is good experimental evidence for the Presence and
Absence theory, and we shall express in terms of it the various cases which
come up for discussion in succeeding chapters.
Rose × Breda
|
+---------+---------+
| |
Duplex × Duplex
Rose | Rose
+-------+------+-------+-------+
| | | | |
Duplex Rose Duplex Single Breda
Rose Single (Duplex
and Simplex)
* * * * *
{42}
CHAPTER V
INTERACTION OF FACTORS
We have now reached a point at which it is possible to formulate a definite
conception of the living organism. A plant or animal is a living entity
whose properties may in large measure be expressed in terms of
unit-characters, and it is the possession of a greater or lesser number of
such unit-characters renders it possible for us to draw sharp distinctions
between one individual and another. These unit-characters are represented
by definite factors in the gamete which in the process of heredity behave
as indivisible entities, and are distributed according to a definite
scheme. The factor for this or that unit-character is either present in the
gamete or it is not present. It must be there in its entirety or completely
absent. Such at any rate is the view to which recent experiment has led us.
But as to the nature of these factors, the conditions under which they
exist in the gamete, and the manner in which they produce their specific
effects in the zygote, we are at present almost completely in the dark.
The case of the fowls' combs opens up the important question of the extent
to which the various factors can {43} influence one another in the zygote.
The rose and the pea factors are separate entities, and each when present
alone produces a perfectly distinct and characteristic effect upon the
single comb, turning it into a rose or a pea as the case may be. But when
both are present in the same zygote their combined effect is to produce the
walnut comb, a comb which is quite distinct from either and in no sense
intermediate between them. The question of the influence of factors upon
one another did not present itself to Mendel because he worked with
characters which affected different parts of the plant. It was unlikely
that the factor which led to the production of colour in the flower would
affect the shape of the pod, or that the height of the plant would be
influenced by the presence or absence of the factor that determined the
shape of the ripe seed. But when several factors can modify the same
structure it is reasonable to suppose that they will influence one another
in the effects which their simultaneous presence has upon the zygote. By
themselves the pea and the rose factors each produce a definite
modification of the single comb, but when both are present in the zygote,
whether as a single or double dose, the modification that results is quite
different to that produced by either when present alone. Thus we are led to
the conception of characters which depend for their manifestation on more
than one factor in the zygote, and in the present chapter we may consider a
few of the {44} phenomena which result from such interaction between
separate and distinct factors.
White × White
|
Red -------------- F_1
|
+----------+
Red White ------- F_2
(9) (7)
One of the most interesting and instructive cases in which the interaction
between separate factors has been demonstrated is a case in the sweet pea.
All white sweet peas breed true to whiteness. And generally speaking the
result of crossing different whites is to produce nothing but whites,
whether in F_1 or in succeeding generations. But there are certain strains
of white sweet peas which when crossed together produce only coloured
flowers. The colour may be different in different cases, though for our
present purpose we may take a case in which the colour is red. When such
reds are allowed to self-fertilise themselves in the normal way and the
seeds sown, the resulting F_2 generation consists of reds and whites, the
former being rather more numerous than the latter in the proportion of
9 : 7. The raising of a further generation from the seeds of these F_2
plants shows that the whites always breed true to whiteness, but that
different reds may behave differently. Some breed true, others give reds
and whites in the ratio 3 : 1, while others, again, give reds and whites in
the ratio 9 : 7. As in the case of the fowls' combs, this case may be
interpreted in terms of the presence and absence of two factors. {45}
White White
AAbb aaBB
/ \ / \
/ \ / \
Ab Ab aB aB gametes of parents
`-------´
Red F_1
AaBb
/ \
/ \
AB AB
Female gametes Ab Ab Male gametes
of F_1 aB aB of F_1
ab ab
Red in the sweet pea results from the interaction of two factors, and
unless these are both present the red colour cannot appear. Each of the
white parents carried one of the two factors whose interaction is necessary
for the production of the red colour, and as a cross between them brings
these two complementary factors together the F_1 plants must all be red. As
this case is of considerable importance for the proper understanding of
much that is to follow, and as it has been completely worked out, we shall
consider it in some detail. Denoting these two colour factors by A and B
respectively we may proceed to follow out the consequences of this cross.
Since all the F_1 plants were red the constitution of the parental whites
must have been AAbb and aaBB respectively, and their gametes consequently
Ab and aB. The constitution of the F_1 plants must, therefore, be AaBb.
Such a plant being heterozygous for two factors produces a series of
gametes of the four kinds AB, Ab, aB, ab, and produces them in equal
numbers (cf. p. 36). To obtain the various types of zygotes which are
produced when such {46} a series of pollen grains meets a similar series of
ovules we may make use of the same "chessboard" system which we have
already adopted in the case of the fowls' combs.
+------+------+------+------+
|AB....|AB....|AB....|AB....|
|AB....|Ab....|aB....|ab....|
|......|......|......|......|
+------+------+------+------+
|Ab....|Ab |Ab....|Ab |
|AB....|Ab |aB....|ab |
|......| |......| |
+------+------+------+------+
|aB....|aB....|aB |aB |
|AB....|Ab....|aB |ab |
|......|......| | |
+------+------+------+------+
|ab....|ab |ab |ab |
|AB....|Ab |aB |ab |
|......| | | |
+------+------+------+------+
FIG. 7.
Diagram to illustrate the nature of the F_2 generation from the two
white sweet peas which give a coloured F_1.
An examination of this figure (Fig. 7) shows that 9 out of the 16 squares
contain both A and B, while 7 contain either A or B alone, or neither. In
other words, on this view of the nature of the two white sweet peas we
should in the F_2 generation look for the appearance of coloured and white
flowers in the ratio 9 : 7. And this, as we have already seen, is what was
actually found by experiment. Further examination of the figure shows that
the coloured plants are not all of the same constitution, but are of four
kinds with respect to their zygotic constitution, viz. AABB, AABb, AaBB,
and AaBb. Since AABB is homozygous for both A and B, all the gametes which
it produces must contain both of these factors, and such a plant must
therefore breed true to the red colour. A plant of the {47} constitution
AABb is homozygous for the factor A, but heterozygous for B. All of its
gametes will contain A, but only one-half of them will contain B, _i.e._ it
produces equal numbers of gametes AB and Ab. Two such series of gametes
coming together must give a generation consisting of x AABB, 2x AABb, and x
AAbb, that is, reds and whites in the ratio 3 : 1. Lastly the red zygotes
of the constitution AaBb have the same constitution as the original red
made from the two whites, and must therefore when bred from give reds and
whites in the ratio 9 : 7. The existence of all these three sorts of reds
was demonstrated by experiment, and the proportions in which they were met
with tallied with the theoretical explanation.
The theory was further tested by an examination into the properties of the
various F_2 whites which come from a coloured plant that has itself been
produced by the mating of two whites. As Fig. 7 shows, these are, in
respect of their constitution, of five different kinds, viz. AAbb, Aabb,
aaBB, aaBb, and aabb. Since none of them produce anything but whites on
self-fertilisation it was found necessary to test their properties in
another way, and the method adopted was that of crossing them together. It
is obvious that when this is done we should expect different results in
different cases. Thus the cross between two whites of the constitution AAbb
and aaBB should give nothing but coloured plants; for these two whites are
of {48} the same constitution as the original two whites from which the
experiment started. On the other hand, the cross between a white of the
constitution aabb and any other white can never give anything but whites.
For no white contains both A and B, or it would not be white, and a plant
of the constitution aabb cannot supply the complementary factor necessary
for the production of colour. Again, two whites of the constitution Aabb
and aaBb when crossed should give both coloured and white flowers, the
latter being three times as numerous as the former. Without going into
further detail it may be stated that the results of a long series of
crosses between the various F_2 whites accorded closely with the
theoretical explanation.
From the evidence afforded by this exhaustive set of experiments it is
impossible to resist the deduction that the appearance of colour in the
sweet pea depends upon the interaction of two factors which are
independently transmitted according to the ordinary scheme of Mendelian
inheritance. What these factors are is still an open question. Recent
evidence of a chemical nature indicates that colour in a flower is due to
the interaction of two definitive substances: (1) a colourless "chromogen,"
or colour basis; and (2) a ferment which behaves as an activator of the
chromogen, and by inducing some process of oxidation, leads to the
formation of a coloured substance. But whether these two bodies exist as
such {49} in the gametes or whether in some other form we have as yet no
means of deciding.
Since the elucidation of the nature of colour in the sweet pea phenomena of
a similar kind have been witnessed in other plants, notably in stocks,
snapdragons, and orchids. Nor is this class of phenomena confined to
plants. In the course of a series of experiments upon the plumage colour of
poultry, indications were obtained that different white breeds did not
always owe their whiteness to the same cause. Crosses were accordingly made
between the white Silky fowl and a pure white strain derived from the white
Dorking. Each of these had been previously shown to behave as a simple
recessive to colour. When the two were crossed only fully coloured birds
resulted. From analogy with the case of the sweet pea it was anticipated
that such F_1 coloured birds when bred together would produce an F_2
generation consisting of coloured and white birds in the ratio 9 : 7, and
when the experiment was made this was actually shown to be the case. With
the growth of knowledge it is probable that further striking parallels of
this nature between the plant and animal worlds will be met with.
Before quitting the subject of these experiments attention may be drawn to
the fact that the 9 : 7 ratio is in reality a 9 : 3 : 3 : 1 ratio in which
the last three terms are indistinguishable owing to the special
circumstances that neither factor can produce a visible effect without {50}
the co-operation of the other. And we may further emphasise the fact that
although the two factors thus interact upon one another they are
nevertheless transmitted quite independently and in accordance with the
ordinary Mendelian scheme.
Agouti × Agouti
|
+---------------+
Agouti × Agouti
|
+---------+---------+
Agouti Black Albino
(9) (3) (4)
One of the earliest sets of experiments demonstrating the interaction of
separate factors was that made by the French zoologist Cuénot on the coat
colours of mice. It was shown that in certain cases agouti, which is the
colour of the ordinary wild grey mouse, behaves as a dominant to the albino
variety, _i.e._ the F_2 generation from such a cross consists of agoutis
and albinos in the ratio 3 : 1. But in other cases the cross between albino
and agouti gave a different result. In the F_1 generation appeared only
agoutis as before, but the F_2 generation consisted of three distinct
types, viz. agoutis, albinos, _and blacks_. Whence the sudden appearance of
the new type? The answer is a simple one. The albino parent was really a
black. But it lacked the factor without which the colour is unable to
develop, and consequently it remained an albino. If we denote this factor
by C, then the constitution of an albino must be cc, while that of a
coloured animal may be CC or Cc, according as to whether it breeds true to
colour or can {51} throw albinos. Agouti was previously known to be a
simple dominant to black, _i.e._ an agouti is a black rabbit plus an
additional greying factor which modifies the black into agouti. This factor
we will denote by G, and we will use B for the black factor. Our original
agouti and albino parents we may therefore regard as in constitution GGCCBB
and ggccBB respectively. Both of the parents are homozygous for black. The
gametes produced by the two parents are GCB, and gcB, and the constitution
of the F_1 animals must be GgCcBB. Being heterozygous for two factors they
will produce four kinds of gametes in equal numbers, viz. GCB, GcB, gCB,
and gcB. The results of the mating of two such similar series of gametes
when the F_1 animals are bred together we may determine by the usual
"chessboard" method (Fig. 8). Out of the 16 squares 9 contain both C and G
in addition to B. Such animals must be agoutis. Three squares contain C but
not G. Such animals must be coloured, but as they do not contain the
modifying agouti factor their colour will be black. The remaining four
squares do not contain C, and in the absence of this colour-developing
factor they must all be albinos. Theory demands that the three classes
agouti, black, and albino should appear in F_2 in the ratio 9 : 3 : 4;
experiment has shown that these are the only classes that appear, and that
the proportions in which they are produced accord closely with the
theoretical expectation. Put briefly, then, the explanation {52} of this
case is that all the animals are black, and that we are dealing with the
presence and absence of two factors, a colour developer (C), and a colour
modifier (G), both acting, as it were, upon a substratum of black. The F_2
generation really consists of the four classes agoutis, blacks, albino
agoutis, and albino blacks in the ratio 9 : 3 : 3 : 1. But since in the
absence of the colour developer C the colour modifier G can produce no
visible result, the last two classes of the ratio are indistinguishable,
and our F_2 generation comes to consist of three classes in the ratio
9 : 3 : 4, instead of four classes in the ratio 9 : 3 : 3 : 1.
+-------+-------+-------+-------+
|GCB....|GCB....|GCB....|GCB....|
|GCB....|GcB....|gCB....|gcB....|
|.......|.......|.......|.......|
|.Agouti|.Agouti|.Agouti|.Agouti|
+-------+-------+-------+-------+
|GcB....|GcB |GcB....|GcB |
|GCB....|GcB |gCB....|gcB |
|.......| |.......| |
|.Agouti| Albino|.Agouti| Albino|
+-------+-------+-------+-------+
|gCB....|gCB....|gCB####|gCB####|
|GCB....|GcB....|gCB####|gcB####|
|.......|.......|#######|#######|
|.Agouti|.Agouti|##BLACK|##BLACK|
+-------+-------+-------+-------+
|gcB....|gcB |gcB####|gcB |
|GCB....|GcB |gCB####|gcB |
|.......| |#######| |
|.Agouti| Albino|##BLACK| Albino|
+-------+-------+-------+-------+
FIG. 8.
Diagram to illustrate the nature of the F_2 generation which may arise
from the mating of agouti with albino in mice or rabbits.
This explanation was further tested by experiments with the albinos. In an
F_2 family of this nature there ought to be three kinds, viz. albinos
homozygous for G (GGccBB), albinos heterozygous for G (GgccBB), and albinos
without G (ggccBB). These albinos are, as it were, like photographic plates
exposed but undeveloped. {53} Their potentialities may be quite different,
although they all look alike, but this can only be tested by treating them
with a colour developer. In the case of the mice and rabbits the
potentiality for which we wish to test is the presence or absence of the
factor G, and in order to develop the colour we must introduce the factor
C. Our developer, therefore, must contain C but not G. In other words, it
must be a homozygous black mouse or rabbit, ggCCBB. Since such an animal is
pure for C it must, when mated with any of the albinos, produce only
coloured offspring. And since it does not contain G the appearance of
agoutis among its offspring must be attributed to the presence of G in the
albino. Tested in this way the F_2 albinos were proved, as was expected, to
be of three kinds: (1) those which gave only agouti, _i.e._ which were
homozygous for G; (2) those which gave agoutis and blacks in approximately
equal numbers, _i.e._ which were heterozygous for G; and (3) those which
gave only blacks, and therefore did not contain G.
Though albinos, whether mice, rabbits, rats, or other animals, breed true
to albinism, and though albinism behaves as a simple recessive to colour,
yet albinos may be of many different sorts. There are in fact just as many
kinds of albinos as there are coloured forms--neither more nor less. And
all these different kinds of albinos may breed together, transmitting the
various colour factors according to the Mendelian scheme of inheritance,
{54} and yet the visible result will be nothing but albinos. Under the mask
of albinism is all the while occurring that segregation of the different
colour factors which would result in all the varieties of coloured forms,
if only the essential factor for colour development were present. But put
in the developer by crossing with a pure coloured form and their variety of
constitution can then at last become manifest.
So far we have dealt with cases in which the production of a character is
dependent upon the interaction of two factors. But it may be that some
characters require the simultaneous presence of a greater number of factors
for their manifestation, and the experiments of Miss Saunders have shown
that there is a character in stocks which is unable to appear except
through the interaction of three distinct factors. Coloured stocks may be
either hoary, with the leaves and stem covered by small hairs, or they may
lack the hairy covering, in which case they are termed glabrous. Hoariness
is dominant to glabrousness; that is to say, there is a definite factor
which can turn the glabrous into a hoary plant when it is present. But in
families where coloured and white stocks occur the white are always
glabrous, while the coloured plants may or may not be hoary. Now colour in
the stock as in the sweet pea has been proved to be dependent upon the
interaction of two separate factors. Hence hoariness depends upon three
separate factors, and a stock cannot be hoary unless {55} it contains the
hoary factor in addition to the two colour factors. It requires the
presence of all these three factors to produce the hoary character, though
how this comes about we have not at present the least idea.
[Illustration: FIG. 9.
Sections of primula flowers. The anthers are shown as black. A, "pin" form
with long style and anthers set low down; B, "thrum" form with short style
and anthers set higher up; C, homostyle form with anthers set low down as
in "pin," but with short style. This form only occurs with the large eye.]
[Illustration: FIG. 10.
Two primula flowers showing the extent of the small and of the large eye.]
A somewhat different and less usual form of interaction between factors may
be illustrated by a case in primulas recently worked out by Bateson and
Gregory. Like the common primrose, the primula exhibits both pin-eyed and
thrum-eyed varieties. In the former the style is long, and the centre of
the eye is formed by the end of the stigma which more or less plugs up the
opening of the corolla (cf. Fig. 9, A); in the latter the style is short
and hidden by the four anthers which spring from higher up in the corolla
and form the centre of the eye (cf. Fig. 9, B). The greater part of the
"eye" is formed by the greenish-yellow patches on each petal just at the
opening {56} of the corolla. In most primulas the eye is small, but there
are some in which it is large and extends as a flush over a considerable
part of the petals (Fig. 10). Experiments showed that these two pairs of
characters behave in simple Mendelian fashion, short style ( = "thrum")
being dominant to long style (= "pin") and small eye dominant to large.
Besides the normal long and short styled forms, there occurs a third form,
which has been termed homostyle. In this form the anthers are placed low
down in the corolla tube as they are in the long-styled form, but the style
remains short instead of reaching up to the corolla opening (Fig. 9, C). In
the course of their experiments Bateson and Gregory crossed a large-eyed
homostyle plant with a small-eyed thrum ( = short style). The F_1 plants
were all short styled with small eyes. {57} On self-fertilisation these
gave an F_2 generation consisting of four types, viz. short styled with
small eyes, short styled with large eyes, _long styled_ with small eyes,
and _homostyled_ with large eyes. The notable feature of this generation is
the appearance of long-styled plants, which, however, occur only in
association with the small eye. The proportions in which these four types
appeared shows that the presence or absence of but two factors is
concerned, and at the same time provides the key to the nature of the
homostyled plants. These are potentially long styled, and the position of
the anthers is that of normal long-styled plants, but owing to some
interaction between the factors the style itself is unable to reach its
full development unless the factor for the small eye is present. For this
reason long-styled plants with the large eye are always of the homostyle
form. What the connecting-link between these apparently unrelated
structures may be we cannot yet picture to ourselves, any more than we can
picture the relation between flower {58} colour and hairiness in stocks. It
is evident, however, that the conception of the interaction of factors,
besides clearing up much that is paradoxical in heredity, promises to
indicate lines of research which may lead to valuable extensions in our
knowledge of the way in which the various parts of the living organism are
related to one another.
Short style } { Homo style
small eye } × { large eye
|
Short style
small eye
|
+-------------+----------+-----------+
Short style Short style Long style Homo style
small eye large eye ("pin") large eye
(9) (3) (3) (1)
* * * * *
{59}
CHAPTER VI
REVERSION
As soon as the idea was grasped that characters in plants and animals might
be due to the interaction of complementary factors, it became evident that
this threw clear light upon the hitherto puzzling phenomenon of reversion.
We have already seen that in certain cases the cross between a black mouse
or rabbit and an albino, each belonging to true breeding strains, might
produce nothing but agoutis. In other words, the cross between the black
and the white in certain instances results in a complete reversion to the
wild grey form. Expressed in Mendelian terms, the production of the agouti
was the necessary consequence of the meeting of the factors C and G in the
same zygote. As soon as they are brought together, no matter in what way,
the reversion is bound to occur. Reversion, therefore, in such cases we may
regard as the bringing together of complementary factors which had somehow
in the course of evolution become separated from one another. In the
simplest cases, such as that of the black and the white rabbit, only two
factors are concerned, and one of them is brought in from each of the {60}
two parents. But in other cases the nature of the reversion may be more
complicated owing to a larger number of factors being concerned, though the
general principle remains the same. Careful breeding from the reversions
will enable us in each case to determine the number and nature of the
factors concerned, and in illustration of this we may take another example
from rabbits. The Himalayan rabbit is a well-known breed. In appearance it
is a white rabbit with pink eyes, but the ears, paws, and nose are black
(Pl. I., 2). The Dutch rabbit is another well-known breed. Generally
speaking, the anterior portion of the body is white, and the posterior part
coloured. Anteriorly, however, the eyes are surrounded by coloured patches
extending up to the ears, which are entirely coloured. At the same time the
hind paws are white (cf. Pl. I., 1). Dutch rabbits exist in many varieties
of colour, though in each one of these the distribution of colour and white
shows the same relations. In the experiments about to be described a yellow
Dutch rabbit was crossed with a Himalaya. The result was a reversion to the
wild agouti colour (Pl. I., 3). Some of the F_1 individuals showed white
patches, while others were self-coloured. On breeding from the F_1 animals
a series of coloured forms resulted in F_2. These were agoutis, blacks,
yellows, and sooty yellows, the so-called tortoise shells of the fancy (Pl.
I., 4-7).
[Illustration: PLATE I.
1, Yellow Dutch Rabbit; 2, Himalayan; 3, Agouti ( = grey) F_1 reversion;
4-8, F_2 types, viz.: 4, Agouti; 5, Yellow; 6, Black; 7, Tortoiseshell; 8,
Himalayan.]
{61}
Yellow × Himalayan
|
+-------------+
Agouti × Agouti
|
+--------+------+-------+----------+
Agouti Yellow Black Tortoise Himalayan
Shell
(27) (9) (9) (3) (16)
In addition to these appeared Himalayans with either black points or with
lighter brownish ones, and the proportions in which they came showed the
Himalayan character to be a simple recessive. A certain number of the
coloured forms exhibited the Dutch marking to a greater or less extent, but
as its inheritance in this set of experiments is complicated and has not
yet been worked out, we may for the present neglect it and confine our
attention to the coloured types and to the Himalayans. The proportion in
which the four coloured types appeared in F_2 was very nearly 9 agoutis, 3
blacks, 3 yellows, and 1 tortoiseshell. Evidently we are here dealing with
two factors: (1) the grey factor (G), which modifies black into agouti, or
tortoiseshell into yellow; and (2) an intensifying factor (I), which
intensifies yellow into agouti and tortoiseshell into black. It may be
mentioned here that other experiments confirmed the view that the yellow
rabbit is a dilute agouti, and the tortoiseshell a dilute black. The
Himalayan pattern behaves as a recessive to self-colour. It is a
self-coloured black rabbit lacking a factor that allows the colour to
develop except in the points. That factor we may denote {62} by X, and as
far as it is concerned the Himalayan is constitutionally xx. The Himalayan
contains the intensifying factor, for such pigment as it possesses in the
points is full coloured. At the same time it is black, _i.e._ lacking in
the factor G. With regard to these three factors, therefore, the
constitution of the Himalayan is ggIIxx. The last character which we have
to consider in this cross is the Dutch character. This was found by Hurst
to behave as a recessive to self-colour (S), and for our present purpose we
will regard it as differing from a self-coloured rabbit in the lack of this
factor.[3] The Himalayan is really a self-coloured animal, which, however,
is unable to show itself as a full black owing to its not possessing the
factor X. The results of breeding experiments then suggest that we may
denote the Himalayan by the formula ggIIxxSS and the yellow Dutch by
GGiiXXss. Each lacks two of the factors upon the full complement of which
the agouti colour depends. By crossing them the complete series GIXS is
brought into the same zygote, and the result is a reversion to the colour
of the wild rabbit.
Bush × Cupid
|
Tall -------------------------- F_1
|
+----------+---+------+----------+
Tall Bush Cupid Cupid -------- F_2
(procumbent) (erect)
Most of the instances of reversion yet worked out are those in which colour
characters are concerned. The sweet pea, however, supplies us with a good
example of reversion in structural characters. A dwarf variety known as the
"Cupid" has been extensively grown for {63} some years. In these little
plants the internodes are very short and the stems are few in number, and
attain to a length of only 9-10 inches. In course of growth they diverge
from one another, and come to lie prostrate on the ground (Pl. II., 2).
Curiously enough, although the whole plant is dwarfed in other respects,
this does not seem to affect the size of the flower, which is that of a
normal sweet pea. Another though less-known variety is the "Bush" sweet
pea. Its name is derived from its habit of growth. The numerous stems do
not diverge from one another, but all grow up side by side, giving the
plant the appearance of a compact bush (Pl. II., 1). Under ordinary
conditions it attains a height of 3½-4 feet. A number of crosses were made
between the Bush and Cupid varieties, with the somewhat unexpected result
that in every instance the F_1 plants showed complete reversion to the size
and habit of the ordinary tall sweet pea (Pl. II., 3), which is the form of
the wild plant as it occurs in Sicily to-day. The F_2 generation from these
reversionary talls consisted of four different types, viz. {64} talls,
bushes, Cupids of the procumbent type like the original Cupid parent, and
Cupids with the compact upright Bush habit (Pl. II., 4). These four types
appeared in the ratio 9 : 3 : 3 : 1, and this, of course, provided the clue
to the nature of the case. The characters concerned are (1) long internode
of stem between the leaves which is dominant to short internode, and (2)
the creeping procumbent habit which is dominant to the erect bush-like
habit. Of these characters length of internode was carried by the Bush, and
the procumbent habit by the original Cupid parent. The bringing of them
together by the cross resulted in a procumbent plant with long internodes.
This is the ordinary tall sweet pea of the wild Sicilian type, reversion
here, again, being due to the bringing together of two complementary
factors which had somehow become separated in the course of evolution.
To this interpretation it may be objected that the ordinary sweet pea is a
plant of upright habit. This, however, is not true. It only appears so
because the conventional way of growing it is to train it up sticks. In
reality it is of procumbent habit, with divergent stems like the ordinary
Cupid, a fact which can easily be observed by anyone who will watch them
grow without the artificial aid of prepared supports.
[Illustration: PLATE II.
1, Bush Sweet Pea; 2, Cupid Sweet Pea; 3, F_1 reversionary Tall; 4, Erect
Cupid Sweet Pea; 5, Purple Invincible; 6, Painted Lady; 7, Duke of
Westminster (hooded standard).]
{65}
The cases of reversion with which we have so far dealt have been cases in
which the reversion occurs as an immediate result of a cross, _i.e._ in the
F_1 generation. This is perhaps the commonest mode of reversion, but
instances are known in which the reversion that occurs when two pure types
are crossed does not appear until the F_2 generation. Such a case we have
already met with in the fowls' combs. It will be remembered that the cross
between pure pea and pure rose gave walnut combs in F_1, while in the F_2
generation a definite proportion, 1 in 16, of single combs appeared (cf. p.
32). Now the single comb is the form that is found in the wild jungle fowl,
which is generally regarded as the ancestor of the domestic breeds. If this
is so, we have a case of reversion in F_2; and this in the _absence_ of the
two factors brought together by the rose-comb and pea-comb parents. Instead
of the reversion being due to the bringing together of two complementary
factors, we must regard it here as due to the association of two
complementary absences. To this question, however, we shall revert later in
discussing the origin of domesticated varieties.
Black Barb × White Fantail Black Barb × Spot[4]
| |
Dark × Dark
Among the offspring one very similar
to the wild blue rock.
Black White
Barb × Fantail
|
+------------------------+
Black × Black
(White Splashed) | (White Splashed)
|
+--------+--------+---------+-----------+
Black Black Blue Blue White
(White Splashed) (White Splashed)
\--------------/ \-------------/
(9) (3) (4)
There is one other instance of reversion to which we must allude. This is
Darwin's famous case of the occasional appearance of pigeons reverting to
the wild blue rock (_Columba livia_), when certain domesticated races are
crossed together. As is well known, Darwin made use of this as an argument
for regarding all the domesticated varieties as having arisen from the same
wild species. The original experiment is somewhat complicated, and is shown
in the accompanying scheme. Essentially it lay in {66} following the
results flowing from crosses between blacks and whites. Experiments
recently made by Staples-Browne have shown that this case of reversion also
can be readily interpreted in Mendelian terms. In these experiments the
cross was made between black barbs and white fantails. The F_1 birds were
all black with some white splashes, evidently due to a separate factor
introduced by the fantail. On breeding these blacks together they gave an
F_2 generation, consisting of blacks (with or without white splashes),
blues (with or without white splashes), and whites in the ratio 9 : 3 : 4.
The factors concerned are colour (C), in the absence of {67} which a bird
is white, and a black modifier (B), in the absence of which a coloured bird
is blue. The original black barb contained both of these factors, being in
constitution CCBB. The fantail, however, contained neither, and was
constitutionally ccbb. The F_1 birds produced by crossing were in
constitution CcBb, and being heterozygous for two factors produced in equal
numbers the four sorts of gametes CB, Cb, cB, cb. The results of two such
series of gametes being brought together are shown in the usual way in Fig.
11. A blue is a bird containing the colour factor but lacking the black
modifier, _i.e._ of the constitution CCbb, or Ccbb, and such birds as the
figure shows appear in the F_2 generation on the average three times out of
sixteen. Reversion here comes about in F_2, when the redistribution of the
factors leads to the formation of zygotes containing one of the two factors
but not the other.
+-------+-------+-------+-------+
|CB#####|CB#####|CB#####|CB#####|
|CB#####|Cb#####|cB#####|cb#####|
|#######|#######|#######|#######|
|##BLACK|##BLACK|##BLACK|##BLACK|
+-------+-------+-------+-------+
|Cb#####|Cb.....|Cb#####|Cb.....|
|CB#####|Cb.....|cB#####|cb.....|
|#######|.......|#######|.......|
|##BLACK|...Blue|##BLACK|...Blue|
+-------+-------+-------+-------+
|cB#####|cB#####|cB |cB |
|CB#####|Cb#####|cB |cb |
|#######|#######| | |
|##BLACK|##BLACK| | |
+-------+-------+-------+-------+
|cb#####|cb.....|cb |cb |
|CB#####|Cb.....|cB |cb |
|#######|.......| | |
|##BLACK|...Blue| | |
+-------+-------+-------+-------+
FIG. 11.
Diagram to illustrate the appearance of the reversionary blue pigeon in
F_2 from the cross of black with white.
* * * * *
{68}
CHAPTER VII
DOMINANCE
[Illustration: FIG. 12.
Primula flowers to illustrate the intermediate nature of the F_1 flower
when _sinensis_ is crossed with _stellata_.]
Sinensis × Stellata
|
Intermediate -------------------------- F_1
|
+---------+----+-------+-----------+
Sinensis Inter. Inter. Stellata --------- F_2
| | |
Sinensis | Stellata --------- F{3}
| +-----+-----+-----+ |
Sinensis Sin. Int. Int. Stell. Stellata --------- F{4}
In the cases which we have hitherto considered the presence of a factor
produces its full effect whether it is introduced by both of the gametes
which go to form the zygote, or by one of them alone. The heterozygous tall
pea or the heterozygous rose-combed fowl cannot be distinguished from the
homozygous form by mere inspection, however close. Breeding tests alone can
decide which is the heterozygous and which the homozygous form. Though this
is true for the majority of characters yet investigated, there are cases
known in which the heterozygous form differs in appearance from either
parent. Among plants such a case has been met with in the primula. The
ordinary Chinese primula (_P. sinensis_) (Fig. 12) has large rather wavy
petals much crenated at the edges. In the Star Primula (_P. stellata_) the
flowers are much smaller, while the petals are flat and present only a
terminal notch instead of the numerous crenations of _P. sinensis_. The
heterozygote produced by crossing these forms is intermediate in size and
appearance. When self-fertilised such plants behave in simple Mendelian
fashion, {69} giving a generation consisting of _sinensis_, intermediates,
and _stellata_ in the ratio 1 : 2 : 1. Subsequent breeding from these
plants showed that both the _sinensis_ and _stellata_ which appeared in the
F_2 generation bred true, while the intermediates always gave all three
forms again in the same proportion. But though there is no dominance of the
character of either parent in such a case as this, the Mendelian principle
of segregation could hardly have a better illustration.
{70}
Blue × Blue
|
+-------------+--------+------------+
Black Blue × Blue White
| | |
| +------+--+--+-----+ |
Black Black Blue Blue White White
| |
Black ------------- × ------------- White
|
Blue
(all)
Among birds a case of similar nature is that of the Blue Andalusian fowl.
Fanciers have long recognised the difficulty of getting this variety to
breed true. Of a slaty blue colour itself with darker hackles and with
black lacing on the feathers of the breast, it always throws "wasters" of
two kinds, viz. blacks, and whites splashed with black. Careful breeding
from the blues shows that the three sorts are always produced in the same
definite {71} proportions, viz., one black, two blues, one splashed white.
This at once suggests that the black and the splashed white are the two
homozygous forms, and that the blues are heterozygous, _i.e._, producing
equal numbers of "black" and "white splashed" gametes. The view was tested
by breeding the "wasters" together--black with black, and splashed white
with splashed white--and it was found that each bred true to its respective
type. But when the black and the splashed white were crossed they gave, as
was expected, nothing but blues. In other words, we have the seeming
paradox of the black and the splashed white producing twice as many blues
as do the blues when bred together. The black and the splashed white
"wasters" are in reality the pure breeds, while the "pure" Blue Andalusian
is a mongrel which no amount of selection will ever be able to fix.
In such cases as this it is obvious that we cannot speak of dominance. And
with the disappearance of this phenomenon we lose one criterion for
determining which of the two parent forms possesses the additional factor.
Are we, for example, to regard the black Andalusian as a splashed white to
which has been added a double dose of a colour-intensifying factor, or are
we to consider the white splashed bird as a black which is unable to show
its true pigmentation owing to the possession of some inhibiting factor
which prevents the manifestation of the black. Either interpretation fits
the facts equally well, {72} and until further experiments have been
devised and carried out it is not possible to decide which is the correct
view.
Besides these comparatively rare cases where the heterozygote cannot be
said to bear a closer resemblance to one parent more than to the other,
there are cases in which it is often possible to draw a visible distinction
between the heterozygote and the pure dominant. There are certain white
breeds of poultry, notably the White Leghorn, in which the white behaves as
a dominant to colour. But the heterozygous whites made by crossing the
dominant white birds with a pure coloured form (such as the Brown Leghorn)
almost invariably show a few coloured feathers or "ticks" in their plumage.
The dominance of white is not quite complete, and renders it possible to
distinguish the pure from the impure dominant without recourse to breeding
experiments.
+------+------+------+------+
|CI |CI |CI |CI |
|CI |Ci |cI |ci |
| | | | |
| | | | |
+------+------+------+------+
|Ci |Ci....|Ci |Ci....|
|CI |Ci....|cI |ci....|
| |......| |......|
| |......| |......|
+------+------+------+------+
|cI |cI |cI |cI |
|CI |Ci |cI |ci |
| | | | |
| | | | |
+------+------+------+------+
|ci |ci....|ci |ci |
|CI |Ci....|cI |ci |
| |......| | |
| |......| | |
+------+------+------+------+
FIG. 13.
Diagram to illustrate the nature of the F_2 generation from the cross
between dominant white and recessive white fowls.
This case of the dominant white fowl opens up another interesting problem
in connection with dominance. By accepting the "Presence and Absence"
hypothesis we are committed to the view that the dominant form possesses an
extra factor as compared with the recessive. The natural way of looking at
this case of the fowl is to regard white as the absence of colour. But were
this so, colour should be dominant to white, which is not the case. We are
therefore forced to suppose that the absence of colour in this instance is
due to the presence of a factor whose {73} property is to inhibit the
production of colour in what would otherwise be a pure coloured bird. On
this view the dominant white fowl is a coloured bird plus a factor which
inhibits the development of the colour. The view can be put to the test of
experiment. We have already seen that there are other white fowls in which
white is recessive to colour, and that the whiteness of such birds is due
to the fact that they lack a factor for the development of colour. If we
denote this factor by C and our postulated inhibitor factor in the dominant
white bird by I, then we must write the constitution of the recessive white
as ccii, and the dominant white as CCII. We may now work out the results we
ought to obtain when a cross is made between these two pure white breeds.
The constitution of the F_1 bird must be CcIi. Such birds being
heterozygous for the inhibitor factor, should be whites showing some
coloured "ticks." Being heterozygous for both of the two factors C and I,
they will produce in equal numbers the four different sorts of gametes CI,
Ci, cI, ci. The result of bringing two such similar series of gametes
together is shown in Fig. 13. Out of the sixteen squares, twelve contain I;
these will be white birds either with or without a few coloured ticks.
Three contain C but not I: these must be coloured birds. One contains
neither C nor I; this must be a white. From such a mating we ought,
therefore, to obtain both white and coloured birds in the ratio 13 : 3. The
results thus theoretically {74} deduced were found to accord with the
actual facts of experiment. The F_1 birds were all "ticked" whites, and in
the F_2 generation came white and coloured birds in the expected ratio.
There seems, therefore, little reason to doubt that the dominant white is a
coloured bird in which the absence of colour is due to the action of a
colour-inhibiting factor, though as to the nature of that factor we can at
present make no surmise. It is probable that other facts, which at first
sight do not appear to be in agreement with the "Presence and Absence"
hypothesis, will eventually be brought into line through the action of
inhibitor factors. Such a case, for instance, is that of bearded and
beardless wheats. Though the beard is obviously the additional character,
the bearded condition is recessive to the beardless. Probably we ought to
regard the beardless as a bearded wheat in which there is an inhibitor that
stops the beard from growing. It is not unlikely that as time goes on we
shall {75} find many more such cases of the action of inhibitor factors,
and we must be prepared to find that the same visible effect may be
produced either by the addition or by the omission of a factor. The
dominant and recessive white poultry are indistinguishable in appearance.
Yet the one contains a factor more and the other a factor less than the
coloured bird.
[Illustration: FIG. 14.
Ears of beardless and bearded wheat. The beardless condition is dominant to
the bearded.]
{76}
A phenomenon sometimes termed irregularity of dominance has been
investigated in a few cases. In certain breeds of poultry such as Dorkings
there occurs an extra toe directed backwards like the hallux (cf. Fig. 15).
In some families this character behaves as an ordinary dominant to the
normal, giving the expected 3 : 1 ratio in F_2. But in other families
similarly bred the proportions of birds with and without the extra toe
appear to be unusual. It has been shown that in such a family some of the
birds without the extra toe may nevertheless transmit the peculiarity when
mated with birds belonging to strains in which the extra toe never occurs.
Though the external appearance of the bird generally affords some
indication of the nature of the gametes which it is carrying, this is not
always the case. Nevertheless we have reason to suppose that the character
segregates in the gametes, though the nature of these cannot always be
decided from the appearance of the bird which bears them.
[Illustration: FIG. 15.
Fowls' feet. On the right a normal and on the left one with an extra toe.]
[Illustration: FIG. 16.
Scheme to illustrate the inheritance of horns in sheep. Heterozygous males
shown dark with a white spot, heterozygous females light with a dark spot
in the centre.]
There are cases in which an apparent irregularity of dominance has been
shown to depend upon another character, as in the experiments with sheep
carried out by Professor Wood. In these experiments two breeds were
crossed, of which one, the Dorset, is horned in both sexes, while the
other, the Suffolk, is without horns in either sex. Whichever way the cross
was made the resulting F_1 generation was similar; the rams were horned,
and {77} the ewes were hornless. In the F_2 generation raised from these
F_1 animals both horned and hornless types appeared in both sexes but in
very different proportions. While the horned rams were about three times as
numerous as the hornless, this relation was reversed among the females, in
which the horned formed only about one-quarter of the total. The simplest
explanation of this interesting case is to suppose that the dominance of
the horned character depends upon the sex of the animal--that it is
dominant in the male but recessive in the female. A pretty experiment was
devised for putting this view to the test. If it is true, equal numbers of
gametes with and without the horned factor must be produced by the F_1
ewes, while the factor should be lacking in all the gametes of the hornless
F_2 rams. A {78} hornless ram, therefore, put to a flock of F_1 ewes should
give rise to equal numbers of zygotes which are heterozygous for the horned
character, and of zygotes in which it is completely absent. And since the
heterozygous males are horned, while the heterozgyous females are hornless,
we should expect from this mating equal numbers of horned and hornless
rams, but only hornless ewes. The result of the experiment confirmed this
expectation. Of the ram lambs 9 were horned and 8 were hornless, while all
the 11 ewe lambs were completely destitute of horns.
[Illustration: PLATE III.
Sheep]
* * * * *
{79}
CHAPTER VIII
WILD FORMS AND DOMESTIC VARIETIES
In discussing the phenomena of reversion we have seen that in most cases
such reversion occurs when the two varieties which are crossed each contain
certain factors lacking in the other, of which the full complement is
necessary for the production of the reversionary wild form. This at once
suggests the idea that the various domestic forms of animals and plants
have arisen by the omission from time to time of this factor or of that. In
some cases we have clear evidence that this is the most natural
interpretation of the relation between the cultivated and the wild forms.
Probably the species in which it is most evident is the sweet pea
(_Lathyrus odoratus_). We have already seen reason to suppose that as
regards certain structural features the Bush variety is a wild lacking the
factor for the procumbent habit, that the Cupid is a wild without the
factor for the long inter-node, and that the Bush Cupid is a wild minus
both these factors. Nor is the evidence less clear for the many colour
varieties. In illustration we may consider in more detail a case in which
the cross between two whites resulted {80} in a complete reversion to the
purple colour characteristic of the wild Sicilian form (Pl. IV.). In this
particular instance subsequent breeding from the purples resulted in the
production of six different colour forms in addition to whites. The
proportion of the coloured forms to the whites was 9 : 7 (cf. p. 44), but
it is with the relation of the six coloured forms that we are concerned
here. Of these six forms three were purples and three were reds. The three
purple forms were (1) the wild bicolor purple with blue wings known in
cultivation as the Purple Invincible (Pl. IV., 4); (2) a deep purple with
purple wings (Pl. IV., 5); and (3) a very dilute purple known as the
Picotee (Pl. IV., 6). Corresponding to these three purple forms were three
reds: (1) a bicolor red known as Painted Lady (Pl. IV., 7); (2) a deep red
with red wings known as Miss Hunt (Pl. IV., 8); and (3) a very pale red
which we have termed Tinged White[5] (Pl. IV., 9). In the F_2 generation
the total number of purples bore to the total number of reds the ratio
3 : 1, and this ratio was maintained for each of the corresponding classes.
Purple, therefore, is dominant to red, and each of the three classes of red
differs from its corresponding purple in not possessing the blue factor (B)
which turns it into purple.
[Illustration: PLATE IV.
1, 2, Emily Henderson; 3, F_1 reversionary Purple; 4-10, Various F_2 forms:
4, Purple; 5, Deep Purple; 6, Picotee; 7, Painted Lady; 8, Miss Hunt; 9,
Tinged White; 10, White.]
{81} Again, the proportion in which the three classes of purples appeared
was 9 bicolors, 3 deep purples, 4 picotees. We are, therefore, concerned
here with the operation of two factors: (1) a light wing factor, which
renders the bicolor dominant to the dark winged form; and (2) a factor for
intense colour, which occurs in the bicolor and in the deep purple, but is
lacking in the dilute picotee. And here it should be mentioned that these
conclusions rest upon an exhaustive set of experiments involving the
breeding of many thousands of plants. In this cross, therefore, we are
concerned with the presence or absence of five factors, which we may denote
as follows:--
A colour base, R.
A colour developer, C.
A purple factor, B.
A light wing factor, L.
A factor for intense colour, I.
On this notation our six coloured forms are:--
(1) Purple bicolor CRBLI.[6]
(2) Deep purple CRBlI.
(3) Picotee CRBLi or CRBli.
(4) Red bicolor ( = Painted Lady) CRbLI.
(5) Deep red ( = Miss Hunt) CRblI.
(6) Tinged white CRbLi or CRbli.
It will be noticed in this series that the various coloured {82} forms can
be expressed by the omission of one or more factors from the purple bicolor
of the wild type. With the complete omission of each factor a new colour
type results, and it is difficult to resist the inference that the various
cultivated forms of the sweet pea have arisen from the wild by some process
of this kind. Such a view tallies with what we know of the behaviour of the
wild form when crossed by any of the garden varieties. Wherever such
crossing has been made the form of the hybrid has been that of the wild,
thus supporting the view that the wild contains a complete set of all the
differentiating factors which are to be found in the sweet pea.
Moreover, this view is in harmony with such historical evidence as is to be
gleaned from botanical literature, and from old seedsmen's catalogues. The
wild sweet pea first reached England in 1699, having been sent from Sicily
by the monk Franciscus Cupani as a present to a certain Dr. Uvedale in the
county of Middlesex. Somewhat later we hear of two new varieties, the red
bicolor, or Painted Lady, and the white, each of which may be regarded as
having "sported" from the wild purple by the omission of the purple factor,
or of one of the two colour factors. In 1793 we find a seedsman offering
also what he called black and scarlet varieties. It is probable that these
were our deep purple and Miss Hunt varieties, and that somewhere about this
time the factor for the {83} light wing (L) was dropped out in certain
plants. In 1860 we have evidence that the pale purple or Picotee, and with
it doubtless the Tinged White, had come into existence. This time it was
the factor for intense colour which had dropped out. And so the story goes
on until the present day, and it is now possible to express by the same
simple method the relation of the modern shades, of purple and reds, of
blues and pinks, of hooded and wavy standards, to one another and to the
original wild form. The constitution of many of these has now been worked
out, and to-day it would be a simple though perhaps tedious task to denote
all the different varieties by a series of letters indicating the factors
which they contain, instead of by the present system of calling them after
kings and queens, and famous generals, and ladies more or less well known.
From what we know of the history of the various strains of sweet peas one
thing stands out clearly. The new character does not arise from a
pre-existing variety by any process of gradual selection, conscious or
otherwise. It turns up suddenly complete in itself, and thereafter it can
be associated by crossing with other existing characters to produce a gamut
of new varieties. If, for example, the character of hooding in the standard
(cf. Pl. II., 7) suddenly turned up in such a family as that shown on Plate
IV. we should be able to get a hooded form corresponding to each of the
forms with the erect {84} standard; in other words, the arrival of the new
form would give us the possibility of fourteen varieties instead of seven.
As we know, the hooded character already exists. It is recessive to the
erect standard, and we have reason to suppose that it arose as a sudden
sport by the omission of the factor in whose presence the standard assumes
the erect shape characteristic of the wild flower. It is largely by keeping
his eyes open and seizing upon such sports for crossing purposes that the
horticulturist "improves" the plants with which he deals. How these sports
or _mutations_ come about we can now surmise. They must owe their origin to
a disturbance in the processes of cell division through which the gametes
originate. At some stage or other the normal equal distribution of the
various factors is upset, and some of the gametes receive a factor less
than others. From the union of two such gametes, provided that they are
still capable of fertilisation, comes the zygote which in course of growth
develops the new character.
Why these mutations arise: what leads to the surmised unequal division of
the gametes: of this we know practically nothing. Nor until we can induce
the production of mutations at will are we likely to understand the
conditions which govern their formation. Nevertheless there are already
hints scattered about the recent literature of experimental biology which
lead us to hope that we may know more of these matters in the future. {85}
In respect of the evolution of its now multitudinous varieties, the story
of the sweet pea is clear and straightforward. These have all arisen from
the wild by a process of continuous loss. Everything was there in the
beginning, and as the wild plant parted with factor after factor there came
into being the long series of derived forms. Exquisite as are the results
of civilization, it is by the degradation of the wild that they have been
brought about. How far are we justified in regarding this as a picture of
the manner in which evolution works?
There are certainly other species in which we must suppose that this is the
way that the various domesticated forms have arisen. Such, for example, is
the case in the rabbit, where most of the colour varieties are recessive to
the wild agouti form. Such also is the case in the rat, where the black and
albino varieties and the various pattern forms are also recessive to the
wild agouti type. And with the exception of a certain yellow variety to
which we shall refer later, such is also the case with the many fancy
varieties of mice.
Nevertheless there are other cases in which we must suppose evolution to
have proceeded by the interpolation of characters. In discussing reversion
on crossing, we have already seen that this may not occur until the F_2
generation, as, for example, in the instance of the fowls' combs (cf. p.
65). The reversion to the single comb occurred as the result of the removal
of the two factors {86} for rose and pea. These two domesticated varieties
must be regarded as each possessing an additional factor in comparison with
the wild single-combed bird. During the evolution of the fowl, these two
factors must be conceived of as having been interpolated in some way. And
the same holds good for the inhibitory factor on which, as we have seen,
the dominant white character of certain poultry depends. In pigeons, too,
if we regard the blue rock as the ancestor of the domesticated breeds, we
must suppose that an additional melanic factor has arisen at some stage.
For we have already seen that black is dominant to blue, and the characters
of F_1, together with the greater number of blacks than blues in F_2,
negatives the possibility that we are here dealing with an inhibitory
factor. The hornless or polled condition of cattle, again, is dominant to
the horned condition, and if, as seems reasonable, we regard the original
ancestors of domestic cattle as having been horned, we have here again the
interpolation of an inhibitory factor somewhere in the course of evolution.
On the whole, therefore, we must be prepared to admit that the evolution of
domestic varieties may come about by a process of addition of factors in
some cases and of subtraction in others. It may be that what we term
additional factors fall into distinct categories from the rest. So far,
experiment seems to show that they are either of the nature of melanic
factors, or of inhibitory {87} factors, or of reduplication factors as in
the case of the fowls' combs. But while the data remain so scanty,
speculation in these matters is too hazardous to be profitable.
* * * * *
{88}
CHAPTER IX
REPULSION AND COUPLING OF FACTORS
Although different factors may act together to produce specific results in
the zygote through their interaction, yet in all the cases we have hitherto
considered the heredity of each of the different factors is entirely
independent. The interaction of the factors affects the characters of the
zygote, but makes no difference to the distribution of the separate
factors, which is always in strict accordance with the ordinary Mendelian
scheme. Each factor in this respect behaves as though the other were not
present.
A few cases have been worked out in which the distribution of the different
factors to the gametes is affected by their simultaneous presence in the
zygote. And the influence which they are able to exert upon one another in
such cases is of two kinds. They may repel one another, refusing, as it
were, to enter into the same zygote, or they may attract one another, and,
becoming linked together, pass into the same gamete, as it were by
preference. For the moment we may consider these two sets of phenomena
apart. {89}
One of the best illustrations of repulsion between factors occurs in the
sweet pea. We have already seen that the loss of the blue or purple factor
(B) from the wild bicolor results in the formation of the red bicolor known
as Painted Lady (Pl. IV., 7). Further, we have seen that the hooded
standard is recessive to the ordinary erect standard. The omission of the
factor for the erect standard (E) from the purple bicolor (Pl. II., 5)
results in a hooded purple known as Duke of Westminster (Pl. II., 7). And
here it should be mentioned that in the corresponding hooded forms the
difference in colour between the wings and standard is not nearly so marked
as in the forms with the erect standard, but the difference in structure
appears to affect the colour, which becomes nearly uniform. This may be
readily seen by comparing the picture of the purple bicolor on Plate II.
with that of the Duke of Westminster flower.
Now when a Duke of Westminster is mated with a Painted Lady the factor for
erect standard (E) is brought in by the red, and that for blue (B) by the
Duke, and the offspring are consequently all purple bicolors. Purples so
formed are all heterozygous for these two factors, and were the case a
simple one, such as those which have already been discussed, we should
expect the F_2 generation to consist of the four forms: erect purple,
hooded purple, erect red, and hooded red in the ratio 9 : 3 : 3 : 1. Such,
however, is not the case. The F_2 generation {90} actually consists of only
three forms, viz. erect red, erect purple, and hooded purple, and the ratio
in which these three forms occur is 1 : 2 : 1. No hooded red has been known
to occur in such a family. Moreover further breeding shows that while the
erect reds and the hooded purples always breed true, the erect purples in
such families _never_ breed true, but always behave like the original F_1
plant, giving the three forms again in the ratio 1 : 2 : 1. Yet we know
that there is no difficulty in getting purple bicolors to breed true from
other families; and we know also that hooded red sweet peas exist in other
strains.
Painted Lady × Duke of Westminster
(erect red) | (hooded purple)
|
Purple Invincible
(erect purple)
|
+-------------+-----------------+
| | |
Painted Purple Invincible Duke of
Lady Westminster
(1) (2) (1)
EEbb eeBB Parents
/\ /\
/ \ / \
/ \ / \
Eb Eb eB eB gametes
\------------/
EeBb F_2
____/ \____
/ \
Fem. gametes of F_1 Eb ---> EEbb <--- Eb Male gametes of F_1
Eb ---> EeBb <--- eB
eB ---> EeBb <--- Eb
eB ---> eeBB <--- eB
\----/
F_2 generation
On the assumption that there exists a repulsion between the factors for
erect standard and blue in a plant which is heterozygous for both, this
peculiar case receives a simple explanation. The constitutions of the erect
red and the hooded purple are EEbb and eeBB respectively and that of the
F_1 erect purple is EeBb. Now let us suppose that in such a zygote there
exists a repulsion {91} between E and B, such that when the plant forms
gametes these two factors will not go into the same gamete. On this view it
can only form two kinds of gametes, viz. Eb and eB, and these, of course,
will be formed in equal numbers. Such a plant on self-fertilisation must
give the zygotic series EEbb + 2 EeBb + eeBB, _i.e._ 1 erect red, 2 erect
purples, and 1 hooded purple. And because the erect reds and the hooded
purples are respectively homozygous for E and B, they must thenceforward
breed true. The erect purples, on the other hand, being always formed by
the union of a gamete Eb with a gamete eB, are always heterozygous for both
of these factors. They can, consequently, never breed true, but must always
give erect reds, erect purples, and hooded purples in the ratio 1 : 2 : 1.
The experimental facts are readily explained on the assumption of repulsion
between the two {92} factors B and E during the formation of the gametes in
a plant which is heterozygous for both.
Other similar cases of factorial repulsion have been demonstrated in the
sweet pea, and two of these are also concerned with the two factors with
which we have just been dealing. Two distinct varieties of pollen grains
occur in this species, viz. the ordinary oblong form and a rather smaller
rounded grain. The former is dominant to the latter.[7] When a cross is
made between a purple with round pollen and a red with long pollen the F_1
plant is a long pollened purple. But the F_2 generation consists of purples
with round pollen, purples with long pollen, and reds with long pollen in
the ratio 1 : 2 : 1. No red with round pollen appears in F_2 owing to
repulsion between the factors for purple (B) and for long pollen (L).
Similarly plants produced by crossing a red hooded long with a red round
having an erect standard give in F_1 long pollened reds with an erect
standard, and these in F_2 produce the three types, round pollened erect,
long pollened erect, and long pollened hooded, in the ratio 1 : 2 : 1. The
repulsion here is between the long pollen factor (L) and the factor for the
erect standard (E).
{93}
Yet another similar case is known in which we are concerned with quite
different factors. In some sweet peas the axils whence the leaves and
flower stalks spring from the main stem are of a deep red colour. In others
they are green. The dark pigmented axil is dominant to the light one.
Again, in some sweet peas the anthers are sterile, setting no pollen, and
this condition is recessive to the ordinary fertile condition. When a
sterile plant with a dark axil is crossed by a fertile plant with a light
axil, the F_1 plants are all fertile with dark axils. But such plants in
F_2 give fertiles with light axils, fertiles with dark axils, and steriles
with dark axils in the ratio 1 : 2 : 1. No light axilled steriles appear
from such a cross owing to the repulsion between the factor for dark axil
(D) and that for the fertile anther (F).
These four cases have already been found in the sweet pea, and similar
phenomena have been met with by Gregory in primulas. To certain seemingly
analogous cases in animals where sex is concerned we shall refer later.
Now all of these four cases present a common feature which probably has not
escaped the attention of the reader. In all of them _the original cross was
such as to introduce one of the repelling factors with each of the two
parents_. If we denote our two factors by A and B, the crosses have always
been of the nature AAbb × aaBB. Let us now consider what happens when both
of the {94} factors, which in these cases repel one another, are introduced
by one of the parents, and neither by the other parent. And in particular
we will take the case in which we are concerned with purple and red flower
colour, and with long and round pollen, _i.e._ with the factors B and L.
When a purple long (BBLL) is crossed with a red round (bbll) the F_1 (BbLl)
is a purple with long pollen, identical in appearance with that produced by
crossing the long pollened red with the round pollened purple. But the
nature of the F_2 generation is in some respects very different. The ratio
of purples to reds and of longs to rounds is in each case 3 : 1, as before.
But instead of an association between the red and the long pollen
characters the reverse is the case. The long pollen character is now
associated with purple and the round pollen with red. The association,
however, is not quite complete, and the examination of a large quantity of
similarly bred material shows that the purple longs are about twelve times
as numerous as the purple rounds, while the red rounds are rather more than
three times as many as the red longs. Now this peculiar result could be
brought about if the gametic series produced by the F_1 plant consisted of
7 BL + 1 Bl + 1 bL + 7 bl out of every 16 gametes. Fertilization between
two such similar series of 16 gametes would result in 256 plants, of which
177 would be purple longs, 15 purple rounds, 15 red longs, and 49 red
rounds--a proportion of the four different kinds very close to {95} that
actually found by experiment. It will be noticed that in the whole family
the purples are to the reds as 3 : 1, and the longs are also three times as
numerous as the rounds. The peculiarity of the case lies in the
distribution of these two characters with regard to one another. In some
way or other the factors for blue and for long pollen become linked
together in the cell divisions that give rise to the gametes, but the
linking is not complete. This holds good for all the four cases in which
repulsion between the factors occurs when one of the two factors is
introduced by each of the parents. _When both of the factors are brought
into the cross by the same parent we get coupling between them instead of
repulsion._ The phenomena of repulsion and coupling between separate
factors are intimately related, though hitherto we have not been able to
suggest why this should be so.
Nor for the present can we suggest why certain factors should be linked
together in the peculiar way that we have reason to suppose that they are
during the process of the formation of the gametes. Nevertheless the
phenomena are very definite, and it is not unlikely that a further study of
them may throw important light on the architecture of the living cell.
APPENDIX TO CHAPTER IX
As it is possible that some readers may care, in spite of its complexity,
to enter rather more fully into the peculiar phenomenon {96} of the
coupling of characters, I have brought together some further data in this
Appendix. In the case we have already considered, where the factors for
blue colour and long pollen are concerned, we have been led to suppose that
the gametes produced by the heterozygous plant are of the nature 7 BL : 1
Bl : 1 bL : 7 bl. Such a series of ovules fertilised by a similar series of
pollen grains will give a generation of the following composition:--
49 BBLL + 7 BBLl + 7 BbLL + 49 BbLl
+ 7 BBLl + 7 BbLL + BbLl
+ BbLl
+ 49 BbLl
\---------------------------------/
177 purple, long
+ BBll + 7 Bbll + bbLL + 7 bbLl + 49 bbll
+ 7 Bbll + 7 bbLl
\-------------/ \-----------/ \-----/
15 purple, 15 red, 49 red,
round long round
and as this theoretical result fits closely with the actual figures
obtained by experiment we have reason for supposing that the heterozygous
plant produces a series of gametes in which the factors are coupled in this
way. The intensity of the coupling, however, varies in different cases.
Where we are dealing with another, viz. fertility (F) and the dark axil
(D), the experimental numbers accord with the view that the gametic series
is here 15 FD : 1 Fd : 1 fD : 15 fd. The coupling is in this instance more
intense. In the case of the erect standard (E) and blueness (B) the
coupling is even more intense, and the experimental evidence available at
present points to the gametic series here being 63 Eb : 1 EB : 1 eB : 63
eb. There is evidence also for supposing that the intensity of the coupling
may vary in different families for the same pair of factors. The coupling
between blue and long pollen is generally on the 7 : 1 : 1 : 7 {97} basis,
but in some cases it may be on the 15 : 1 : 1 : 15 basis. But though the
intensity of the coupling may vary it varies in an orderly way. If A and B
are the two factors concerned, the results obtained in F_2 are explicable
on the assumption that the ratio of the four sorts of gametes produced is a
term of the series--
3 AB + Ab + aB + 3 ab
7 AB + Ab + aB + 7 ab
15 AB + Ab + aB + 15 ab, etc., etc.
In such a series the number of gametes containing A is equal to the number
lacking A, and the same is true for B. Consequently the number of zygotes
formed containing A is three times as great as the number of zygotes which
do not contain A; and similarly for B. The proportion of dominants to
recessives in each case is 3 : 1. It is only in the distribution of the
characters with relation to one another that these cases differ from a
simple Mendelian case.
As the study of these series presents another feature of some interest, we
may consider it in a little more detail. In the accompanying table are set
out the results produced by these different series of gametes. The series
marked by an asterisk have already been demonstrated experimentally. The
first term in the series, {98} in which all the four kinds of gametes are
produced in equal numbers is, of course, that of a simple Mendelian case
where no coupling occurs.
+-------+------------------+---------+---------------------------------+
|No. of | Distribution of | No. of | |
|Gametes|Factors in Gametic| Zygotes | Form of F_2 Generation. |
| in | Series |produced.| |
|series.| | | |
+-------+------------------+---------+---------------------------------+
| | AB. Ab. aB. ab. | | AB. Ab. aB. ab. |
| 4 | 1: 1: 1: 1 | 16 | 9 3 3 1 |
| 8 | 3: 1: 1: 3 | 64 | 49 7 7 9 |
| 16 | 7: 1: 1: 7 | 256 | 177 15 15 49* |
| 32 | 15: 1: 1: 15 | 1024 | 737 31 31 225* |
| 64 | 31: 1: 1: 31 | 4096 | 3009 63 63 961 |
| 128 | 63: 1: 1: 63 | 16384 | 12161 127 127 3969* |
| 2n |(n-1): 1: 1:(n-1) | 4n^2 |3n^2-(2n-1) 2n-1 2n-1 n^2-(2n-1)|
+-------+------------------+---------+---------------------------------+
Now, as the table shows, it is possible to express the gametic series by a
general formula (n + 1) AB + Ab + aB + (n - 1) ab, where 2n is the total
number of the gametes in the series. A plant producing such a series of
gametes gives rise to a family of zygotes in which 3n^2 - (2n - 1) show
both of the dominant characters and n^2 - (2n - 1) show both of the
recessive characters, while the number of the two classes which each show
one of the two dominants is (2n - 1). When in such a series the coupling
becomes closer the value of n increases, but in comparison with n^2 its
value becomes less and less. The larger n becomes the more negligible is
its value relatively to n^2. If, therefore, the coupling were very close,
the series 3n^2 - (2n - 1) : (2n - 1) : (2n - 1) : n^2 - (2n - 1) would
approximate more and more to the series 3n^2 : n^2, _i.e._ to a simple
3 : 1 ratio. Though the point is probably of more theoretical than
practical interest, it is not impossible that some of the cases which have
hitherto been regarded as following a simple 3 : 1 ratio will turn out on
further analysis to belong to this more complicated scheme.
* * * * *
{99}
CHAPTER X
SEX
[Illustration: FIG. 17.
_Abraxas grossulariata_, the common currant moth, and (on the right) its
paler lacticolor variety.]
In their simplest expression the phenomena exhibited by Mendelian
characters are sharp and clean cut. Clean cut and sharp also are the
phenomena of sex. It was natural, therefore, that a comparison should have
been early instituted between these two sets of phenomena. As a general
rule, the cross between a male and a female results in the production of
the two sexes in approximately equal numbers. The cross between a
heterozygous dominant and a recessive also leads to equal numbers of
recessives and of heterozygous dominants. Is it not, therefore, possible
that one of the sexes is heterozygous for a factor which is lacking in the
other, and that the presence or absence of this factor determines the sex
of the zygote? The results of some recent experiments would appear to
justify this interpretation, at any rate in particular cases. Of these, the
simplest is that of the common currant moth (_Abraxas grossulariata_), of
which there exists a pale variety (Fig. 17) known as _lacticolor_. The
experiments of Doncaster and Raynor showed that the variety behaved as a
simple recessive to the normal form. But the distribution of the dominants
and {100} recessives [Illustration]with with regard to the sexes was
peculiar. The original cross was between a _lacticolor_ female and a normal
male. All the F_1 moths of both sexes were of the normal _grossulariata_
type. The F_1 insects were then paired together and gave a generation
consisting of 3 normals : 1 _lacticolor_. But all the _lacticolor_ were
females, and all the males were of the normal pattern. It was, however,
found possible to obtain the _lacticolor male_ by mating a _lacticolor_
female with the F_1 male. The family resulting from this cross consisted of
normal males and normal females, _lacticolor_ males and _lacticolor_
females, and the {101} four sorts were produced in approximately equal
numbers. In such a family there was no special association of either of the
two colour varieties with one sex rather than the other. But the reverse
cross, F_1 female by _lacticolor_ male, gave a very different result. As in
the previous cross such families contained equal numbers of the normal form
and of the recessive variety. But all of the normal _grossulariata_ were
males, while all the _lacticolor_ were females. Now this seemingly complex
collection of facts is readily explained if we make the following three
assumptions:--
[Illustration]
(1) The _grossulariata_ character (G) is dominant to the lacticolor
character (g). This is obviously justified by the experiments, for, leaving
the sex distribution out of account, we get the expected 3 : 1 ratio from
F_1 × F_1, and also the expected ratio of equality when the heterozygote is
crossed with the recessive.
(2) The female is heterozygous for a dominant factor (F) which is lacking
in the male. The constitution of a female is consequently Ff, and of a male
ff. This assumption is in harmony with the fact that the sexes are produced
in approximately equal numbers.
(3) There exists repulsion between the factors G and F in a zygote which is
heterozygous for them both. Such zygotes (FfGg) must always be females, and
on this assumption will produce gametes Fg and fG in equal numbers. {102}
[Illustration: FIG. 18.
Scheme of inheritance in the F_1 and F_2 generations resulting from the
cross of _lacticolor_ female with _grossulariata_ male. The character of
each individual is represented by the sex signs in brackets, the black
being _grossulariata_ in appearance and the light ones _lacticolor_.]
We may now construct a scheme for comparison with that on page 100 to show
how these assumptions explain the experimental results. The original
parents were _lacticolor_ female and _grossulariata_ male, which on our
assumptions must be Ffgg and ffGG respectively in constitution. Since the
female is always heterozygous for F, her gametes must be of two kinds, viz.
Fg and fg, while those of the pure _grossulariata_ male must be all fG.
When an ovum Fg is fertilised by a spermatozoon fG, the resulting zygote,
FfGg, is heterozygous for both F and G, and in appearance is a female
_grossulariata_. The zygote resulting from the fertilisation of an ovum fg
by a spermatozoon fG is heterozygous for G, but does not contain F, and
therefore is a male _grossulariata_. Such a male being in constitution
{103} ffGg must produce gametes of two kinds, fG and fg, in equal numbers.
And since we are assuming repulsion between F and G, the F_1 female being
in constitution FfGg, must produce equal numbers of gametes Fg and fG. For
on our assumption F and G cannot enter into the same gamete. The series of
gametes produced by the F_1 moths, therefore, are fG, fg by the male and
Fg, fG by the female. The resulting F_2 generation consequently consists of
the four classes of zygotes Ffgg, FfGg, ffGg, and ffGG in equal numbers. In
other words, the sexes are produced in equal numbers, the proportion of
normal grossulariata to _lacticolor_ is 3 : 1, and all of the _lacticolor_
are females; that is to say, the results worked out on our assumptions
accord with those actually produced by experiment. We may now turn to the
results which should be obtained by crossing the F_1 moths with the
_lacticolor_ variety. And first we will take the cross _lacticolor_ female
× F_1 male. The gametes produced by the lacticolor female we have already
seen to be Fg and fg, while those produced by the F_1 male are fG and fg.
The bringing together of these two series of gametes must result in equal
numbers of the four kinds of zygotes FfGg, Ffgg, ffGg, and ffgg, _i.e._ of
female _grossulariata_ and _lacticolor_, and of male _grossulariata_ and
_lacticolor_ in equal numbers. Here, again, the calculated results accord
with those of experiment. Lastly, we may examine what should happen when
the F_1 female is crossed with the _lacticolor_ {104} male. The F_1 female,
owing to the repulsion between F and G, produces only the two kinds of ova
Fg and fG, and produces them in equal numbers. Since the _lacticolor_ male
can contain neither F nor G, all of its spermatozoa must be fg. The results
of such a cross, therefore, should be to produce equal numbers of the two
kinds of zygote Ffgg and ffGg, _i.e._ of _lacticolor_ females and of
_grossulariata_ males. And this, as we have already seen, is the actual
result of such a cross.
Before leaving the currant moth we may allude to an interesting discovery
which arose out of these experiments. The _lacticolor_ variety in Great
Britain is a southern form and is not known to occur in Scotland. Matings
were made between wild Scotch females and _lacticolor_ males. The families
resulting from such matings were precisely the same as those from
_lacticolor_ males and F_1 females, viz. _grossulariata_ males and
_lacticolor_ females only. We are, therefore, forced to regard the
constitution of the wild _grossulariata_ female as identical with that of
the F_1 female, _i.e._ as heterozygous for the _grossulariata_ factor as
well as for the factor for femaleness. Though from a region where
_lacticolor_ is unknown, the "pure" wild _grossulariata_ female is
nevertheless a permanent mongrel, but it can never reveal its true colours
unless it is mated with a male which is either heterozygous for G or pure
_lacticolor_. And as all the wild northern males are {105} pure for the
_grossulariata_ character this can never happen in a state of nature.
[Illustration: FIG. 19.
Scheme illustrating the result of crossing a Silky hen with a Brown Leghorn
cock. Black sex signs denote deeply pigmented birds, and light sex signs
those without pigmentation. The light signs with a black dot in the centre
denote birds with a small amount of pigment.]
An essential feature of the case of the currant moth lies in the different
results given by reciprocal crosses. _Lacticolor_ female × _grossulariata_
male gives _grossulariata_ alone of both sexes. But _grossulariata_ female
× _lacticolor_ male gives only _grossulariata_ males and _lacticolor_
females. Such a difference between reciprocal crosses has also been found
in other animals, and the experimental results, though sometimes more
complicated, are explicable on the same lines. An interesting case in which
three factors are concerned has been recently worked out in poultry. The
Silky breed of fowls is characterised among other peculiarities by a
remarkable abundance of melanic pigment. The skin is dull black, while the
comb and wattles are of a deep purple colour contrasting sharply with the
white plumage (Pl. V., 3). Dissection shows that this black pigment is
widely spread throughout the body, being especially marked in such
membranes as the mesenteries, the periosteum, and the pia mater surrounding
the brain. It also occurs in the connective tissues among the muscles. In
the Brown Leghorn, on the other hand, this pigment is not found. Reciprocal
crosses between these two breeds gave a remarkable difference in result. A
cross between the Silky hen and the Brown Leghorn cock produced F_1 birds
in which both sexes exhibited only traces of the pigment. On casual
observation they might have {106} passed for unpigmented birds, for with
the exception of an occasional fleck of pigment their skin, comb and
wattles were as clear as in the Brown Leghorn (Pl. V., 1 and 4). Dissection
revealed the presence of a slight amount of internal pigment. Such birds
bred together gave some offspring with the full pigmentation of the Silky,
some without any pigment, and others showing different degrees of pigment.
None of the F_2 male birds, however, showed the full deep pigmentation of
the Silky.
[Illustration: FIG. 20.
Scheme illustrating the result of crossing a Brown Leghorn hen with a Silky
cock (cf. Fig. 19).]
When, however, the cross was made the other way, viz. Brown Leghorn hen ×
Silky cock, the result was different. While the F_1 male birds were almost
destitute of pigment as in the previous cross, the F_1 hens, on the other
hand, were nearly as deeply pigmented as the pure Silky {107} (Pl. V., 2).
The male Silky transmitted the pigmentation, but only to his daughters.
Such birds bred together gave an F_2 generation containing chicks with the
full deep pigment, chicks without pigment, and chicks with various grades
of pigmentation, all the different kinds in both sexes.
[Illustration: FIG. 21.
Scheme to illustrate the result of crossing F_1 birds (_e.g._ Brown Leghorn
× Silky) with the pure Brown Leghorn.]
In analysing this complicated case many other different crosses were made,
but for the present it will be sufficient to mention but one of these, viz.
that between the F_1 birds and the pure Brown Leghorn. The cross between
the F_1 hen and the Brown Leghorn cock produced only birds with a slight
amount of pigment and birds without pigment. And this was true for both the
deeply pigmented and the slightly pigmented types of F_1 hen. But when the
F_1 cock was mated to a Brown Leghorn hen, a definite proportion of the
chicks, one in eight, was deeply pigmented, and _these deeply pigmented
birds were always females_ (cf. Fig. 21). And in this respect all the F_1
males behaved alike, whether they were from the Silky hen or from the Silky
cock. We have, therefore, the paradox that the F_1 hen, though herself
deeply pigmented, cannot transmit this condition to any of her offspring
when she is mated to the unpigmented Brown Leghorn, but that, when
similarly mated, the F_1 cock can transmit this pigmented condition to a
quarter of his female offspring though he himself is almost devoid of
pigment.
[Illustration: PLATE V.
1, 2, F_1 Cock and Hen, ex Brown Leghorn Hen × Silky Cock; 3, Silky Cock;
4, Hen ex Silky Hen × Brown Leghorn Cock.]
{108}
[Illustration: FIG. 22.
Scheme to illustrate the nature of the F_1 generation from the Silky hen
and Brown Leghorn cock (cf. Fig. 23).]
Now all these apparently complicated results, as well as many others to
which we have not alluded, can be expressed by the following simple scheme.
There are three factors affecting pigment, viz. (1) a pigmentation factor
(P); (2) a factor which inhibits the production of pigment (I); and (3) a
factor for femaleness (F), for which the female birds are heterozygous, but
which is not present in the males. Further, we make the assumptions (a)
that there is repulsion between F and I in the female zygote (FfIi), and
(b) that the male Brown Leghorn is homozygous for the inhibitor factor (I),
but that the hen Brown Leghorn is always heterozygous for this factor just
in the same way as the female of the currant moth is always heterozygous
for the _grossulariata_ factor. We may now proceed to show how this
explanation fits the experimental facts which we have given.
The Silky is pure for the pigmentation factor, but does not contain the
inhibitor factor. The Brown Leghorn, on the other hand, contains the
inhibitor factor, but not the {109} pigmentation factor. In crossing a
Silky hen with a Brown Leghorn cock we are mating two birds of the
constitution FfPPii and ffppII, and all the F_1 birds are consequently
heterozygous for both P and I. In such birds the pigment is almost but not
completely suppressed, and as both sexes are of the same constitution with
regard to these two factors they are both of similar appearance.
[Illustration: FIG. 23.
Scheme to illustrate the nature of the F_1 generation from the Brown
Leghorn hen and Silky cock (cf. Fig. 22).]
In the reciprocal cross, on the other hand, we are mating a Silky male
(ffPPii) with a Brown Leghorn hen which on our assumption is heterozygous
for the inhibitor factor (I), and in constitution therefore is FfppIi.
Owing to the repulsion between F and I the gametes produced by such a bird
are Fpi and fpI in equal numbers. All the gametes produced by the Silky
cock are fPi. Hence the constitution of the F_1 male birds produced by this
cross is ffPpIi as before, but the female birds must be all of the
constitution FfPpii. The Silky cock transmits the fully pigmented condition
to his daughters, because the gametes of the Brown Leghorn hen which
contain the factor for femaleness do not contain the {110} inhibitory
factor owing to the repulsion between these factors. The nature of the F_2
generation in each case is in harmony with the above scheme. As, however,
it serves to illustrate certain points in connection with intermediate
forms we shall postpone further consideration of it till we discuss these
matters, and for the present shall limit ourselves to the explanation of
the different behaviour of the F_1 males and females when crossed with the
Brown Leghorn. And, first, the cross of Brown Leghorn female by F_1 male.
The Brown Leghorn hen is on our hypothesis FfppIi, and produces gametes Fpi
and fpI. The F_1 cock is on our hypothesis ffPpIi, and produces in equal
numbers the four kinds of gametes fPI, fPi, fpI, fpi. The result of the
meeting of these two series of gametes is given in Fig. 24. Of the eight
different kinds of zygote formed only one contains P in the absence of I,
and this is a female. The result, as we have already seen, is in accordance
with the experimental facts.
[Illustration: FIG. 24.
Diagram showing the nature of the offspring from a Brown Leghorn hen and an
F_1 cock bred from Silky hen × Brown Leghorn cock, or _vice versa_.]
On the other hand, the Brown Leghorn cock is on our hypothesis ffppII. All
his gametes consequently contain the inhibitor factor, and when he is mated
with an F_1 {111} hen all the zygotes produced must contain I. None of his
offspring, therefore, can be fully pigmented, for this condition only
occurs in the absence of the inhibitor factor among zygotes which are
either homozygous or heterozygous for P.
[Illustration: FIG. 25.
Scheme to illustrate the heterozygous nature of the pure Brown Leghorn hen.
For explanation see text.]
The interpretation of this case turns upon the constitution of the Brown
Leghorn hen, upon her heterozygous condition with regard to the two factors
F and I, and upon the repulsion that occurs between them when the gametes
are formed. Through an independent set of experiments this view of the
nature of the Brown Leghorn hen has been confirmed in an interesting way.
There are fowls which possess neither the factor for pigment nor the
inhibitory factor, which are in constitution ppii. Such birds when crossed
with the Silky give dark pigmented birds of both sexes in F_1, and the F_2
generation consists of pigmented and unpigmented in the ratio 3 : 1. Now a
cock of such a strain crossed with a Brown Leghorn hen should give only
completely unpigmented birds. But if, as we have supposed, the Brown
Leghorn hen is producing gametes Fpi and fpI, the male birds produced by
such a cross should be heterozygous for I, {112} _i.e._ in constitution
ffppIi, while the hen birds, though identical in appearance so far as
absence of pigmentation goes, should not contain this factor but should be
constitutionally Ffppii. Crossed with the pure Silky, the F_1 birds of
opposite sexes should give an entirely different result. For while the hens
should give only deeply pigmented birds of both sexes, the cocks should
give equal numbers of deeply pigmented and slightly pigmented birds (cf.
Fig. 25). These were the results which the experiment actually gave, thus
affording strong confirmation of the view which we have been led to take of
the Brown Leghorn hen. Essentially the poultry case is that of the currant
moth. It differs in that the factor which {113} repels femaleness produces
no visible effect, and its presence or absence can only be determined by
the introduction of a third factor, that for pigmentation.
This conception of the nature of the Brown Leghorn hen leads to a curious
paradox. We have stated that the Silky cock transmits the pigmented
condition, but transmits it to his daughters only. Apparently the case is
one of unequal transmission by the father. Actually, as our analysis has
shown, it is one of unequal transmission by the mother, the father's
contribution to the offspring being identical for each sex. The mother
transmits to the daughters her dominant quality of femaleness, but to
balance this, as it were, she transmits to her sons another quality which
her daughters do not receive. It is a matter of common experience among
human families that in respect to particular qualities the sons tend to
resemble their mothers more than the daughters do, and it is not improbable
that such observations have a real foundation for which the clue may be
provided by the Brown Leghorn hen.
Nor is this the only reflection that the Brown Leghorn suggests. Owing to
the repulsion between the factors for femaleness and for pigment
inhibition, it is impossible by any form of mating to make a hen which is
homozygous for the inhibitor factor. She has bartered away for femaleness
the possibility of ever receiving a double dose of this factor. We know
that in some cases, as, for example, {114} that of the blue Andalusian
fowl, the qualities of the individual are markedly different according as
to whether he or she has received a single or a double dose of a given
factor. It is not inconceivable that some of the qualities in which a man
differs from a woman are founded upon a distinction of this nature. Certain
qualities of intellect, for example, may depend upon the existence in the
individual of a double dose of some factor which is repelled by femaleness.
If this is so, and if woman is bent upon achieving the results which such
qualities of intellect imply, it is not education or training that will
help her. Her problem is to get the factor on which the quality depends
into an ovum that carries also the factor for femaleness.
* * * * *
{115}
CHAPTER XI
SEX (_continued_)
The cases which we have considered in the last chapter belong to a group in
which the peculiarities of inheritance are most easily explained by
supposing that the female is heterozygous for some factor that is not found
in the male. Femaleness is an additional character superposed upon a basis
of maleness, and as we imagine that there is a separate factor for each the
full constitutional formula for a female is FfMM, and for a male ffMM. Both
sexes are homozygous for the male element, and the difference between them
is due to the presence or absence of the female element F.
There are, however, other cases for which the explanation will not suffice,
but can be best interpreted on the view that the male is heterozygous for a
factor which is not found in the female. Such a case is that recently
described by Morgan in America for the pomace fly (_Drosophila
ampelophila_). Normally this little insect has a red eye, but white eyed
individuals are known to occur as rare sports. Red eye is dominant to
white. In their relation to sex the eye colours of the pomace fly {116} are
inherited on the same lines as the _grossulariata_ and _lacticolor_
patterns of the currant moth, but with one essential difference. The factor
which repels the red-eye factor is in this case to be found in the male,
and here consequently it is the male which must be regarded as heterozygous
for a sex factor that is lacking in the female.
[Illustration]
In order to bring these cases and others into line an interesting
suggestion has recently been put forward by Bateson. On this suggestion
each sex is heterozygous for its own sex factor only, and does not contain
the factor proper to the opposite sex. The male is of the constitution,
Mmff and the female Ffmm. Each sex produces two sorts of gametes, Mf and mf
in the case of the male, and Fm, fm in that of the female. But on this view
a further supposition is necessary. If each of the two kinds of spermatozoa
were capable of fertilising each of the two kinds of ova, we should get
individuals of the constitution MmFf and mmff, as well as the normal males
and females, Mmff and Ffmm. As the facts of ordinary bisexual reproduction
afford us no grounds for assuming the existence of these two classes of
individuals, whatever they may be, we must suppose that fertilisation. is
productive only between the spermatozoa carrying M and the ova without F,
or between the spermatozoa {117} without M and the ova containing F. In
other words we must on this view suppose that fertilisations between
certain forms of gametes, even if they can occur, are incapable of giving
rise to zygotes with the capacity for further development. If we admit this
supposition, the scheme just given will cover such cases as those of the
currant moth and the fowl, equally as well as that of the pomace fly. In
the former there is repulsion between either the _grossulariata_ factor and
F, or else between the pigment inhibitor factor and F, while in the latter
there is repulsion between the factor for red eye and M.
[Illustration: FIG. 26.
Scheme to illustrate the probable mode of inheritance of colour-blindness.
The dark signs represent affected individuals. A black dot in the centre
denotes an unaffected female who is capable of transmitting the condition
to her sons.]
Whatever the merits or demerits of such a scheme it certainly does offer an
explanation of a peculiar form of sex limited inheritance in man. It has
long been a matter of common knowledge that colour-blindness is much more
common among men than among women, and also that unaffected women can
transmit it to their sons. At first sight the case is not unlike that of
the sheep, where the horned character is apparently dominant in the male
but recessive in the female. The hypothesis that the colour-blind condition
is due to the presence of an extra factor as compared with the normal, and
that a single dose of it will produce {118} colour-blindness in the male
but not in the female, will cover a good many of the observed facts (cf.
Fig. 26). Moreover, it serves to explain the remarkable fact that _all_ the
sons of colour-blind women are also colour-blind. For a woman cannot be
colour-blind unless she is homozygous for the colour-blind factor, in which
case all her children must get a single dose of it even if she marries a
normal male. And this is sufficient to produce colour-blindness in the
male, though not in the female.
But there is one notable difference in this case as compared with that of
the sheep. When crossed with pure hornless ewes the heterozygous horned ram
transmits the horned character to half his male offspring (cf. p. 71). But
the heterozygous colour-blind man does not behave altogether like a sheep,
for he apparently does not transmit the colour-blind condition to any of
his male offspring. If, however, we suppose that the colour-blind factor is
repelled by the factor for maleness, the amended scheme will cover the
observed facts. For, denoting the colour-blind factor by X, the gametes
produced by the colour-blind male are of two sorts only, viz. Mfx and mfX.
If he marries a normal woman (Ffmmxx), the spermatozoa Mfx unite with ova
fmx to give normal males, while the spermatozoa mfX unite with ova Fmx to
give females which are heterozygous for the colour-blind factor. These
daughters are themselves normal, but transmit the condition to about half
their sons. {119}
The attempt to discover a simple explanation of the nature of sex has led
us to assume that certain combinations between gametes are incapable of
giving rise to zygotes which can develop further. In the various cases
hitherto considered there is no reason to suppose that anything of the sort
occurs, or that the different gametes are otherwise than completely fertile
one with another. One peculiar case, however, has been known for several
years in which some of the gametes are apparently incapable of uniting to
produce offspring. Yellow in the mouse is dominant to agouti, but hitherto
a homozygous yellow has never been met with. The yellows from families
where only yellows and agoutis occur produce, when bred together, yellows
and agoutis in the ratio 2 : 1. If it were an ordinary Mendelian case the
ratio should be 3 : 1, and one out of every three yellows so bred should be
homozygous and give only yellows when crossed with agouti. But Cuénot and
others have shown that _all_ of the yellows are heterozygous, and when
crossed with agoutis give both yellows and agoutis. We are led, therefore,
to suppose that an ovum carrying the yellow factor is unproductive if
fertilised by a spermatozoon which also bears this factor. In this way
alone does it seem possible to explain the deficiency of yellows and the
absence of homozygous ones in the families arising from the mating of
yellows together. At present, however, it remains the only definite
instance among animals in which we have {120} grounds for assuming that
anything in the nature of unproductive fertilisation takes place.[8]
If we turn from animals to plants we find a more complicated state of
affairs. Generally speaking, the higher plants are hermaphrodite, both
ovules and pollen grains occurring on the same flower. Some plants, however
like most animals, are of separate sexes, a single plant bearing only male
or female flowers. In other plants the separate flowers are either male or
female, though both are borne on the same individual. In others, again, the
conditions are even more complex, for the same plant may bear flowers of
three kinds, viz. male, female, and hermaphrodite. Or it may be that these
three forms occur in the same species but in different individuals--female
and hermaphrodites in one species; males, females, and hermaphrodites in
another. One case, however, must be mentioned as it suggests a possibility
which we have not hitherto encountered. In the common English bryony
(_Bryonia dioica_) the sexes are separate, some plants having only male and
others only female flowers. In another European species, _B. alba_, both
male and female flowers occur on the same plant. Correns crossed these two
species reciprocally, and also fertilised _B. dioica_ by its own male with
the following results:--
{121}
dioica [female] × dioica [male] gave [female] [female] and [male] [male]
" × alba [male] " [female] [female] only
alba [female] × dioica [male] " [female] [female] and [male] [male].
The point of chief interest lies in the striking difference shown by the
reciprocal crosses between _dioica_ and _alba_. Males appear when _alba_ is
used as the female parent but not when the female _dioica_ is crossed by
male _alba_. It is possible to suggest more than one scheme to cover these
facts, but we may confine ourselves here to that which seems most in accord
with the general trend of other cases. We will suppose that in _dioica_
femaleness is dominant to maleness, and that the female is heterozygous for
this additional factor. In this species, then, the female produces equal
numbers of ovules with and without the female factor, while this factor is
absent in all the pollen grains. _Alba_ [female] × _dioica_ [male] gives
the same result as _dioica_ [female] × _dioica_ [male], and we must
therefore suppose that alba produces male and female ovules in equal
numbers. _Alba_ [male] x _dioica_ [female], however, gives nothing but
females. Unless, therefore, we assume that there is selective fertilisation
we must suppose that all the pollen grains of alba carry the female
factor--in other words, that so far as the sex factors are concerned there
is a difference between the ovules and pollen grains borne by the same
plant. Unfortunately further investigation of this case is rendered
impossible owing to the complete sterility of the F_1 plants. {122}
[Illustration: FIG. 27.
Single and double stocks raised from the same single parent.]
That the possibility of a difference between the ovules and pollen grains
of the same individual must be taken into account in future work there is
evidence from quite a different source. The double stock is an old
horticultural favourite, and for centuries it has been known that of itself
it sets no seed, but must be raised from special strains of the single
variety. "You must understand withall," wrote John Parkinson of his
gilloflowers,[9] "that those plants that beare double flowers, doe beare no
seed at all ... but the onely way to have double flowers any yeare is to
save the seedes of those plants of this kinde that beare single flowers,
for from that seede will rise some that will beare single, and some double
flowers." With regard to the nature of these double-throwing strains of
singles, Miss Saunders has recently brought out some interesting facts. She
crossed the double-throwing singles with pure singles belonging to strains
in which doubles never occur. The cross was made both ways, and in both
cases all the F_1 plants were single. A distinction, however, appeared when
a further generation was raised from the F_1 plants. All the F_1 plants
from the pollen of the double-throwing single behaved like double-throwing
singles, but of the F_1 plants from the ovules of the double throwers some
behaved as double throwers, and some as pure singles. We are led to infer,
therefore, that the ovules and pollen grains {123} of the double throwers,
though both produced by the same plant, differ in their relation to the
factor (or factors) for doubleness. Doubleness is apparently carried by all
the pollen grains of such plants, but only by some of the ovules. Though
the nature of doubleness in stocks is not yet clearly understood, the facts
discovered by Miss Saunders suggest strongly that the ovules and pollen
grains of the same plant may differ in their transmitting properties,
probably owing to some process of segregation in the growing plant which
leads to an unequal distribution of some or other factors to the cells
which give rise to the ovules as compared with those from which {124} the
pollen grains eventually spring. Whether this may turn out to be the true
account or not, the possibility must not be overlooked in future work.
Single
|
+-------------+
Single Double
/ \
Pollen of × Ovule Pollen × Ovule of
pure single | | pure single
| |
+----------+ |
Single Single Single
| | |
| +-------+ +---------+
Single Single Double Single Double
| | |
| +-------+ +---------+
Single Single Double Single Double
From all this it is clear enough that there is much to be done before the
problem of sex is solved even so far as the biologist can ever expect to
solve it. The possibilities are many, and many a fresh set of facts is
needed before we can hope to decide among them. Yet the occasional glimpses
of clear-cut and orderly phenomena, which Mendelian spectacles have already
enabled us to catch, offer a fair hope that some day they may all be
brought into focus, and assigned their proper places in a general scheme
which shall embrace them all. Then, though not till then, will the problem
of the nature of sex pass from the hands of the biologist into those of the
physicist and the chemist.
* * * * *
{125}
CHAPTER XII
INTERMEDIATES
So far as we have gone we have found it possible to express the various
characters of animals and plants in terms of definite factors which are
carried by the gametes, and are distributed according to a definite scheme.
Whatever may be the nature of these factors it is possible for purposes of
analysis to treat them as indivisible entities which may or may not be
present in any given gamete. When the factor is present it is present as a
whole. The visible properties developed by a zygote in the course of its
growth depend upon the nature and variety of the factors carried in by the
two gametes which went to its making, and to a less degree upon whether
each factor was brought in by both gametes or by one only. If the given
factor is brought in by one gamete only, the resulting heterozygote may be
more or less intermediate between the homozygous form with a double dose of
the factor and the homozygous form which is entirely destitute of the
factor. Cases in point are those of the primula flowers and the Andalusian
fowls. Nevertheless these intermediates produce only pure gametes, as is
{126} shown by the fact that the pure parental types appear in a certain
proportion of their offspring. In such cases as these there is but a single
type of intermediate, and the simple ratio in which this and the two
homozygous forms appear renders the interpretation obvious. But the nature
of the F_2 generation may be much more complex, and, where we are dealing
with factors which interact upon one another, may even present the
appearance of a series of intermediate forms grading from the condition
found in one of the original parents to that which occurred in the other.
As an illustration we may consider the cross between the Brown Leghorn and
Silky fowls which we have already dealt with in connection with the
inheritance of sex. The offspring of a Silky hen mated with a Brown Leghorn
are in both sexes birds with but a trace of the Silky pigmentation. But
when such birds are bred together they produce a generation consisting of
chicks as deeply pigmented as the original Silky parent, chicks devoid of
pigment like the Brown Leghorn, and chicks in which the pigmentation shows
itself in a variety of intermediate stages. Indeed from a hundred chicks
bred in this way it would be possible to pick out a number of individuals
and arrange them in an apparently continuous series of gradually increasing
pigmentation, with the completely unpigmented at one end and the most
deeply pigmented at the other. Nevertheless, the case is one in which
complete segregation of the different factors takes {127}
[Illustration]place, place, and the apparently continuous series of
intermediates is the result of the interaction of the different factors
upon one another. The constitution of the F_1 [male] is a ffPpIi, and such
a bird produces in equal numbers the four sorts of gametes fPI, fPi, fpI,
fpi. The constitution of the F_1 [female] in this case is FfPpIi. Owing to
the repulsion between F and I she produces the four kinds of gametes FPi,
Fpi, fPI, fpi, and produces them in equal numbers. The result of bringing
two such series of gametes together is shown in Fig. 28. Out of the sixteen
types of zygote formed one (FfPPii) is homozygous for the pigmentation
factor, and does not contain the inhibitor factor. Such a bird is as deeply
pigmented as the pure Silky parent. Two, again, contain a single dose of P
in the absence of I. These are nearly as dark as the pure Silky. Four
zygotes are destitute of P, though they may or may not contain I. These
birds are completely devoid of pigment like the Brown Leghorn. The
remaining nine zygotes show {128} various combinations of the two factors P
and I, being either PPIi, PPII, PpII, or PpIi, and in each of these cases
the pigment is more or less intense according to the constitution of the
bird. Thus a bird of the constitution PPIi approaches in pigmentation a
bird of the constitution Ppii, while a bird of the constitution PpII has
but little more pigment than the unpigmented bird. In this way we have
seven distinct grades of pigmentation, and the series is further
complicated by the fact that these various grades exhibit a rather
different amount of pigmentation according as they occur in a male or a
female bird, for, generally speaking, the female of a given grade exhibits
rather more pigment than the corresponding male. The examination of a
number of birds bred in this way might quite well suggest that in this case
we were dealing with a character which could break up, as it were, to give
a continuous series of intergrading forms between the two extremes. With
the constant handling of large numbers it becomes possible to recognise
most of the different grades, though even so it is possible to make
mistakes. Nevertheless, as breeding tests have amply shown, we are dealing
with but two interacting factors which segregate cleanly from one another
according to the strict Mendelian rule. The approach to continuity in
variation exhibited by the F_2 generation depends upon the fact that these
two factors interact upon one another, and to different degrees according
as the zygote is for one {129} or other or both of them in a homozygous or
a heterozygous state. Moreover, certain of these intermediates will breed
true to an intermediate condition of the pigmentation. A male of the
constitution ffPPII when bred with females of the constitution FfPPIi will
produce only males like itself and females like the maternal parent. We
have dealt with this case in some detail, because the existence of families
showing a series of intermediate stages between two characters has
sometimes been brought forward in opposition to the view that the
characters of organisms depend upon specific factors which are transmitted
according to the Mendelian rule. But, as this case from poultry shows
clearly, neither the existence of such a continuous series of
intermediates, nor the fact that some of them may breed true to the
intermediate condition, are incompatible with the Mendelian principle of
segregation.
[Illustration: FIG. 28.
Diagram to illustrate the nature and composition of the F_2 generations
arising from the cross of Silky hen with Brown Leghorn cock.]
In connection with intermediates a more cogent objection to the Mendelian
view is the case of the first cross between two definite varieties
thenceforward breeding true. The case that will naturally occur to the
reader is that of the mulatto, which results from the cross between the
negro and the white. According to general opinion, these mulattos, of
intermediate pigmentation, continue to produce mulattos. Unfortunately this
interesting case has never been critically investigated, and the statement
that the mulatto breeds true rests almost entirely upon {130} information
that is general and often vague. It may be that the inheritance of skin
pigmentation in this instance is a genuine exception to the normal rule,
but at the same time it must not be forgotten that it may be one in which
several interacting factors are concerned, and that the pure white and the
pure black are the result of combinations which from their rarity are apt
to be overlooked. But until we are in possession of accurate information it
is impossible to pronounce definitely upon the nature of the inheritance in
this case.
[Illustration: FIG. 29.
Pedigree of a family which originated from a cross between a Hindu and a
European. Black signs denote individuals as dark as average Hindus. Plain
signs denote quite-fair members, while those with a dot in the centre are
intermediate.]
{131}
On the other hand, from the cross between the darkly pigmented Eastern
races and the white segregation seems to occur in subsequent generations.
Families are to be found in which one parent is a pure white, while the
other has arisen from the cross between the dark and light in the first or
some subsequent generation. Such families may contain children
indistinguishable from pure blonds as well as children of very dark and of
intermediate shades. As an example, I may give the following pedigree,
which was kindly communicated to me by an Anglo-Indian friend (Fig. 29).
The family had resided in England for several generations, so that in this
case there was no question of a further admixture of black. Most noticeable
is the family produced by a very dark lady who had married a white man.
Some of the children were intermediate in colour, but two were fair whites
and two were dark as dark Hindus. This sharp segregation or splitting out
of blacks and whites in addition to intermediates strongly suggests that
the nature of the inheritance is Mendelian, though it may be complicated by
the existence of several factors which may also react upon one another. Nor
must it be forgotten that in so far as these different factors are
concerned the whites themselves may differ in constitution without showing
any trace of it in their appearance. Before the case can be regarded as
settled all these different possibilities will have to be definitely
tested. With the dark Eastern races as with the negro we cannot {132} hope
to come to any conclusion until we have evidence collected by critical and
competent observers.
Though for the present we must regard the case of the negro as not proven,
there are nevertheless two others in which the heredity would appear not to
follow the Mendelian rule. Castle in America crossed the lop-eared rabbit
with the normal form, and found that the F_1 animals were intermediate with
respect to their ears. And subsequent experiment showed that, on the whole,
they bred true to this intermediate condition. The other case relates to
Lepidoptera. The speckled wood butterfly (_Pararge egeria_) has a southern
form which differs from the northern one in the greater brightness and
depth of its yellow-brown markings. The northern form is generally
distinguished as var. _egeriades_. Bateson crossed the southern form from
the south of France with the paler British form, and found that the
offspring were more or less intermediate in colour, and that in subsequent
generations the parental types did not recur. These cases at present stand
alone. It is possible that further research may reveal complications which
mask or interfere with an underlying process of segregation. Or it may be
that segregation does not occur owing to some definite physiological reason
which at present we do not understand.
And here it is impossible not to recall Mendel's own experiences with the
Hawkweeds (_Hieracium_). This {133} genus of plants exhibits an
extraordinary profusion of forms differing from one another sometimes in a
single feature, sometimes in several. The question as to how far these
numerous forms were to be classified as distinct species, how far as
varieties, and how far as products of chance hybridisation, was even at
that time a source of keen controversy among botanists. There is little
doubt that Mendel undertook his experiments on the Hawkweeds in the hope
that the conception of unit-characters so brilliantly demonstrated for the
pea would serve to explain the great profusion of forms among the
Hieraciums. Owing to the minute size of their florets, these plants offer
very considerable technical difficulties in the way of cross fertilisation.
By dint of great perseverance and labour, however, Mendel succeeded in
obtaining a few crosses between different forms. These hybrids were reared
and a further generation produced from them, and, no doubt somewhat to
Mendel's chagrin, every one of them proved to breed true. There was a
complete absence of that segregation of characters which he had shown to
exist in peas and beans, and had probably looked forward with some
confidence to finding in _Hieracium_. More than thirty years passed before
the matter was cleared up. To-day we know that the peculiar behaviour of
the hybrid Hieraciums is due to the fact that they normally produce seed by
a peculiar process of parthenogenesis. It is possible to take an unopened
flower and to shear off with a {134} razor all the male organs together
with the stigmata through which the pollen reaches the ovules. The flower,
nevertheless, sets perfectly good seed. But the cells from which the seeds
develop are not of the same nature as the normal ovules of a plant. They
are not gametes but retain the double structure of the maternal cells. They
are rather to be regarded as of the nature of buds which early become
detached from the parent stock to lead an independent existence, and, like
buds, they reproduce exactly the maternal characteristics. The discovery of
the true nature of this case was only rendered possible by the development
of the study of cytology, and it was not given to Mendel to live long
enough to learn why his hybrid Hieraciums all bred true.
* * * * *
{135}
CHAPTER XIII
VARIATION AND EVOLUTION
Through the facts of heredity we have reached a new conception of the
individual. Hitherto we have been accustomed to distinguish between the
members of a family of rabbits like that illustrated on Plate I. by
assigning to each an individuality, and by making use of certain external
features, such as the coat colour or the markings, as convenient outward
signs to express our idea that the individuality of these different animals
is different. Apart from this, our notions as to what constituted the
individuality in each case were at best but vague. Mendelian analysis has
placed in our hands a more precise method of estimating and expressing the
variations that are to be found between one individual and another. Instead
of looking at the individual as a whole, which is in some vague way endowed
with an individuality marking it off from its fellows, we now regard it as
an organism built up of definite characters superimposed on a basis beyond
which for the moment our analysis will not take us. We have begun to
realise that each individual has a definite architecture, and that this
architecture depends {136} primarily upon the number and variety of the
factors that existed in the two gametes that went to its building. Now most
species exhibit considerable variation and exist in a number, often very
large, of more or less well-defined varieties. How far can this great
variety be explained in terms of a comparatively small number of factors if
the number of possible forms depends upon the number of the factors which
may be present or absent?
In the simple case where the homozygous and heterozygous conditions are
indistinguishable in appearance the number of possible forms is 2, raised
to the power of the number of factors concerned. Thus where one factor is
concerned there are only 2^1 = 2 possible forms, where ten factors are
concerned there are 2^{10} = 1024 possible forms differing from one another
in at most ten and at least one character. Where the factors interact upon
one another this number will, of course, be considerably increased. If the
heterozygous form is different in appearance from the homozygous form,
there are three possible forms connected with each factor; for ten such
factors the possible number of individuals would be 3^{10} = 59,049; for
twenty such factors the possible number of different individuals would be
3^{20} = 3,486,784,401. The presence or absence of a comparatively small
number of factors in a species carries with it the possibility of an
enormous range of individual variation. But every one of these individuals
has a perfectly definite constitution which can {137} be determined in each
case by the ordinary methods of Mendelian analysis. For in every instance
the variation depends upon the presence or absence of definite factors
carried in by the gametes from whose union the individual results. And as
these factors separate out cleanly in the gametes which the individual
forms, such variations as depend upon them are transmitted strictly
according to the Mendelian scheme. Provided that the constitution of the
gametes is unchanged, the heredity of such variation is independent of any
change in the conditions of nutrition or environment which may operate upon
the individual producing the gametes.
But, as everybody knows, an individual organism, whether plant or animal,
reacts, and often reacts markedly, to the environmental conditions under
which its life is passed. More especially is this to be seen where such
characters as size or weight are concerned. More sunlight or a richer soil
may mean stronger growth in a plant, better nutrition may result in a finer
animal, superior education may lead to a more intelligent man. But although
the changed conditions produce a direct effect upon the individual, we have
no indisputable evidence that such alterations are connected with
alterations in the nature of the gametes which the individual produces. And
without this such variations cannot be perpetuated through heredity, but
the conditions which produce the effect must always be renewed in each
{138} successive generation. We are led, therefore, to the conclusion that
two sorts of variations exist, those which are due to the presence of
specific factors in the organism and those which are due to the direct
effect of the environment during its lifetime. The former are known as
_mutations_, and are inherited according to the Mendelian scheme; the
latter have been termed _fluctuations_, and at present we have no valid
reason for supposing that they are ever inherited. For though instances may
be found in which effects produced during the lifetime of the individual
would appear to affect the offspring, this is not necessarily due to
heredity. Thus plants which are poorly nourished and grown under adverse
conditions may set seed from which come plants that are smaller than the
normal although grown under most favorable conditions. It is natural to
attribute the smaller size of the offspring to the conditions under which
the parents were grown, and there is no doubt that we should be quite right
in doing so. Nevertheless, it need have nothing to do with heredity. As we
have already pointed out, the seed is a larval plant which draws its
nourishment from the mother. The size of the offspring is affected because
the poorly nourished parent offered a bad environment to the young plant,
and not because the gametes of the parent were changed through the adverse
conditions under which it grew. The parent in this case is not only the
producer of gametes, but also a part of the environment of the young {139}
plant, and it is in this latter capacity that it affects its offspring.
Wherever, as in plants and mammals, the organism is parasitic upon the
mother during its earlier stages, the state of nutrition of the latter will
almost certainly react upon it, and in this way a semblance of transmitted
weakness or vigour is brought about. Such a connection between mother and
offspring is purely one of environment, and it cannot be too strongly
emphasised that it has nothing to do with the ordinary process of heredity.
The distinction between these two kinds of variation, so entirely different
in their causation, renders it possible to obtain a clearer view of the
process of evolution than that recently prevalent. As Darwin long ago
realised, any theory of evolution must be based upon the facts of heredity
and variation. Evolution only comes about through the survival of certain
variations and the elimination of others. But to be of any moment in
evolutionary change a variation must be inherited. And to be inherited it
must be represented in the gametes. This, as we have seen, is the case for
those variations which we have termed mutations. For the inheritance of
fluctuations, on the other hand, of the variations which result from the
direct action of the environment upon the individual, there is no
indisputable evidence. Consequently we have no reason for regarding them as
playing any part in the production of that succession of temporarily stable
forms which we term evolution. In {140} the light of our present knowledge
we must regard the mutation as the basis of evolution--as the material upon
which natural selection works. For it is the only form of variation of
whose heredity we have any certain knowledge.
It is evident that this view of the process of evolution is in some
respects at variance with that generally held during the past half century.
There we were given the conception of an abstract type representing the
species, and from it most of the individuals diverged in various
directions, though, generally speaking, only to a very small extent. It was
assumed that any variation, however small, might have a selection value,
that is to say, could be transmitted to the offspring. Some of these would
possess it in a less and some in a greater degree than the parent. If the
variation were a useful one, those possessing to a rather greater extent
would be favoured through the action of natural selection at the expense of
their less fortunate brethren, and would leave a greater number of
offspring, of whom some possessed it in an even more marked degree than
themselves. And so it would go on. The process was a cumulative one. The
slightest variation in a favourable direction gave natural selection a
starting-point to work on. Through the continued action of natural
selection on each successive generation the useful variation was gradually
worked up, until at last it reached the magnitude of a specific {141}
distinction. Were it possible in such a case to have all the forms before
us, they would present the appearance of a long series imperceptibly
grading from one extreme to the other.
Upon this view are made two assumptions not unnatural in the absence of any
exact knowledge of the nature of heredity and variation. It was assumed, in
the first place that variation was a continuous process, and, second, that
any variation could be transmitted to the offspring. Both of these
assumptions have since been shown to be unjustified. Even before Mendel's
work became known Bateson had begun to call attention to the prevalence of
discontinuity in variation, and a few years later this was emphasised by
the Dutch botanist Hugo de Vries in his great work on _The Mutation
Theory_. The ferment of new ideas was already working in the solution, and
under the stimulus of Mendel's work they have rapidly crystallised out.
With the advent of heredity as a definite science we have been led to
revise our views as to the nature of variation, and consequently in some
respects as to the trend of evolution. Heritable variation has a definite
basis in the gamete, and it is to the gamete, therefore, not to the
individual, that we must look for the initiation of this process. Somewhere
or other in the course of their production is added or removed the factor
upon whose removal or addition the new variation owes its existence. The
new variation springs into being by a {142} sudden step, not by a process
of gradual and almost imperceptible augmentation. It is not continuous but
discontinuous, because it is based upon the presence or absence of some
definite factor or factors--upon discontinuity in the gametes from which it
sprang. Once formed, its continued existence is subject to the arbitrament
of natural selection. If of value in the struggle for existence natural
selection will decide that those who possess it shall have a better chance
of survival and of leaving offspring than those who do not possess it. If
it is harmful to the individual natural selection will soon bring about its
elimination. But if the new variation is neither harmful nor useful there
seems no reason why it should not persist.
In this way we avoid a difficulty that beset the older view. For on that
view no new character could be developed except by the piling up of minute
variations through the action of natural selection. Consequently any
character found in animals and plants must be supposed to be of some
definite use to the individual. Otherwise it could not have developed
through the action of natural selection. But there are plenty of characters
to which it is exceedingly difficult to ascribe any utility, and the
ingenuity of the supporters of this view has often been severely taxed to
account for their existence. On the more modern view this difficulty is
avoided. The origin of a new variation is independent of natural {143}
selection, and provided that it is not directly harmful there is no reason
why it should not persist. In this way we are released from the burden of
discovering a utilitarian motive behind all the multitudinous characters of
living organisms. For we now recognise that the function of natural
selection is selection and not creation. It has nothing to do with the
formation of the new variation. It merely decides whether it is to survive
or to be eliminated.
One of the arguments made use of by supporters of the older view is that
drawn from the study of adaptation. Animals and plants are as a rule
remarkably well adapted to living the life which their surroundings impose
upon them, and in some cases this adaptation is exceedingly striking.
Especially is this so in the many instances of what is called protective
coloration, where the animal comes to resemble its surroundings so closely
that it may reasonably be supposed to cheat even the keenest sighted enemy.
Surely, we are told, such perfect adaptation could hardly have arisen
through the mere survival of chance sports. Surely there must be some
guiding hand moulding the species into the required shape. The argument is
an old one. For John Ray that guiding hand was the superior wisdom of the
Creator: for the modern Darwinian it is Natural Selection controlling the
direction of variation. Mendelism certainly offers no suggestion of any
such controlling force. It interprets the {144} variations of living forms
in terms of definite physiological factors, and the diversity of animal and
plant life is due to the gain or loss of these factors, to the origination
of new ones, or to fresh combinations among those already in existence. Nor
is there any valid reason against the supposition that even the most
remarkable cases of resemblance, such as that of the leaf insect, may have
arisen through a process of mutation. Experience with domestic plants and
animals shows that the most bizarre forms may arise as sports and
perpetuate themselves. Were such forms, arising under natural conditions,
to be favoured by natural selection owing to a resemblance to something in
their environment we should obtain a striking case of protective
adaptation. And here it must not be forgotten that those striking cases to
which our attention is generally called are but a very small minority of
the existing forms of life.
For that special group of adaptation phenomena classed under the head of
Mimicry, Mendelism seems to offer an interpretation simpler than that at
present in vogue. This perhaps may be more clearly expressed by taking a
specific case. There is in Africa a genus of Danaine butterflies known as
_Amauris_, and there are reasons for considering that the group to which it
belongs possesses properties which render it unpalatable to vertebrate
enemies such as birds or monkeys. In the same region is also found the
genus _Euralia_ belonging to the entirely {145} different family of the
Nymphalidae, to which there is no evidence for assigning the disagreeable
properties of the Danaines. Now the different species of _Euralia_ show
remarkably close resemblances to the species of _Amauris_, which are found
flying in the same region, and it is supposed that by "mimicking" the
unpalatable forms they impose upon their enemies and thereby acquire
immunity from attack. The point at issue is the way in which this seemingly
purposeful resemblance has been brought about.
One of the species of _Euralia_ occurs in two very distinct forms (Pl.
VI.), which were previously regarded as separate species under the names
_E. wahlbergi_ and _E. mima_. These two forms respectively resemble
_Amauris dominicanus_ and _A. echeria_. For purposes of argument we will
assume _A. echeria_ to be the more recent form of the two. On the modern
Darwinian view certain individuals of _A. dominicanus_ gradually diverged
from the _dominicanus_ type and eventually reached the _echeria_ type,
though why this should have happened does not appear to be clear. At the
same time those specimens which tended to vary in the direction of _A.
echeria_ in places where this species was more abundant than _A.
dominicanus_ were encouraged by natural selection, and under its guiding
hand the form _mima_ eventually arose from _wahlbergi_.
According to Mendelian views, on the other hand, {146} _A. echeria_ arose
suddenly from _A. dominicanus_ (or _vice versa_), and similarly _mima_
arose suddenly from _wahlbergi_. If _mima_ occurred where _A. echeria_ was
common and _A. dominicanus_ was rare, its resemblance to the more plentiful
distasteful form would give it the advantage over _wahlbergi_ and allow it
to establish itself in place of the latter. On the modern Darwinian view
natural selection gradually shapes _wahlbergi_ into the _mima_ form owing
to the presence of _A. echeria_; on the Mendelian view natural selection
merely conserves the _mima_ form when once it has arisen. Now this case of
mimicry is one of especial interest, because we have experimental evidence
that the relation between _mima_ and _wahlbergi_ is a simple Mendelian one,
though at present it is uncertain which is the dominant and which the
recessive form. The two have been proved to occur in families bred from the
same female without the occurrence of any intermediates, and the fact that
the two segregate cleanly is strong evidence in favour of the Mendelian
view. On this view the genera _Amauris_ and _Euralia_ contain a similar set
of pattern factors, and the conditions, whatever they may be, which bring
about mutation in the former lead to the production of a similar mutation
in the latter. Of the different forms of _Euralia_ produced in any region
that one has the best chance of survival, through the operation of natural
selection, which resembles the most plentiful _Amauris_ form. Mimetic
resemblance is a true phenomenon, but natural selection plays the part of a
conservative, not of a formative agent.
[Illustration: PLATE VI.]
{147}
It is interesting to recall that in earlier years Darwin was inclined to
ascribe more importance to "sports" as opposed to continuous minute
variation, and to consider that they might play a not inconsiderable part
in the formation of new varieties in nature. This view, however, he gave up
later, because he thought that the relatively rare sport or mutation would
rapidly disappear through the swamping effects of crossing with the more
abundant normal form, and so, even though favoured by natural selection,
would never succeed in establishing itself. Mendel's discovery has
eliminated this difficulty. For suppose that the sport differed from the
normal in the loss of a factor and were recessive. When mated with the
normal this character would seem to disappear, though, of course, half of
the gametes of its progeny would bear it. By continual crossing with
normals a small proportion of heterozygotes would eventually be scattered
among the population, and as soon as any two of these mated together the
recessive sport would appear in one quarter of their offspring.
A suggestive contribution to this subject was recently made by G. H. Hardy.
Considering the distribution of a single factor in a mixed population
consisting of the heterozygous and the two homozygous forms he showed that
such a population breeding at random rapidly fell into a {148} stable
condition with regard to the proportion of these three forms, whatever may
have been the proportion of the three forms to start with. Let us suppose
for instance, that the population consists of p homozygotes of one kind, r
homozygotes of the other kind, and 2 q heterozygotes. Hardy pointed out
that, other things being equal, such a population would be in equilibrium
for this particular factor so long as the condition q^2 = pr was fulfilled.
If the condition is fulfilled to start with, the population remains in
equilibrium. If the condition is not fulfilled to start with, Hardy showed
that a position of equilibrium becomes established after a single
generation, and that this position is thereafter maintained. The
proportions of the three classes which satisfy the equation q^2 = pr are
exceedingly numerous, and populations in which they existed in the
proportions shown in the appended table would remain in stable equilibrium
generation after generation:--
p. 2q. r.
1 2 1
1 4 4
1 6 9
1 8 16
1 20,000 100,000,000
1 2n n^2
This, of course, assumes that all three classes are equally fertile, and
that no form of selection is taking place to the {149} benefit of one class
more than of another. Moreover, it makes no difference whether p represents
the homozygous dominants or whether it stands for the recessives. A
population containing a very small proportion of dominants and one
containing a similar proportion of recessives are equally stable. The term
dominant is in some respects apt to be misleading, for a dominant character
cannot in virtue of its dominance establish itself at the expense of a
recessive one. Brown eyes in man are dominant to blue, but there is no
reason to suppose that as years go on the population of these islands will
become increasingly brown eyed. Given equality of conditions both are on an
equal footing. If, however, either dominant or recessive be favoured by
selection the conditions are altered, and it can be shown that even a small
advantage possessed by the one will rapidly lead to the elimination of the
other. Even with but a 5 per cent selection advantage in its favour it can
be shown that a rare sport will oust the normal form in a few hundred
generations. In this way we are freed from a difficulty inherent in the
older view that varieties arose through a long-continued process involving
the accumulation of very slight variations. On that view the establishing
of a new type was of necessity a very long and tedious business, involving
many thousands of generations. For this reason the biologist has been
accustomed to demand a very large supply of time, often a great deal more
than the physicist is {150} disposed to grant, and this has sometimes led
him to expostulate with the latter for cutting off the supply. On the newer
views, however, this difficulty need not arise, for we realise that the
origin and establishing of a new form may be a very much more rapid process
than has hitherto been deemed possible.
One last question with regard to evolution. How far does Mendelism help us
in connection with the problem of the origin of species? Among the plants
and animals with which we have dealt we have been able to show that
distinct differences, often considerable, in colour, size, and structure,
may be interpreted in terms of Mendelian factors. It is not unlikely that
most of the various characters which the systematist uses to mark off one
species from another, the so-called specific characters, are of this
nature. They serve as convenient labels, but are not essential to the
conception of species. A systematist who defined the wild sweet pea could
hardly fail to include in his definition such characters as the procumbent
habit, the tendrils, the form of the pollen, the shape of the flower, and
its purple colour. Yet all these and other characters have been proved to
depend upon the presence of definite factors which can be removed by
appropriate crossing. By this means we can produce a small plant a few
inches in height with an erect habit of growth, without tendrils, with
round instead of oblong pollen, and with colourless deformed flowers quite
different {151} in appearance from those of the wild form. Such a plant
would breed perfectly true, and a botanist to whom it was presented, if
ignorant of its origin, might easily relegate it to a different genus.
Nevertheless, though so widely divergent in structure, such a plant must
yet be regarded as belonging to the species _Lathyrus odoratus_. For it
still remains fertile with the many different varieties of sweet pea. It is
not visible attributes that constitute the essential difference between one
species and another. The essential difference, whatever it may be, is that
underlying the phenomenon of sterility. The visible attributes are those
made use of by the systematist in cataloguing the different forms of animal
and plant life, for he has no other choice. But it must not be forgotten
that they are often misleading. Until they were bred together _Euralia
wahlbergi_ and _E. mima_ were regarded as perfectly valid species, and
there is little doubt that numbers of recognised species will eventually
fall to the ground in the same way as soon as we are in a position to apply
the test of breeding. Mendelism has helped us to realise that specific
characters may be but incidental to a species--that the true criterion of
what constitutes a species is sterility, and that particular form of
sterility which prevents two healthy gametes on uniting from producing a
zygote with normal powers of growth and reproduction. For there are forms
of sterility which are purely mechanical. The pollen of _Mirabilis jalapa_
cannot fertilise _M._ {152} _longiflora_, because the pollen tubes of the
former are not long enough to penetrate down to the ovules of the latter.
Hybrids can nevertheless be obtained from the reciprocal cross. Nor should
we expect offspring from a St. Bernard and a toy terrier without recourse
to artificial fertilisation. Or sterility may be due to pathological causes
which prevent the gametes from meeting one another in a healthy state. But
in most cases it is probable that the sterility is due to some other cause.
It is not inconceivable that definite differences in chemical composition
render the protoplasm of one species toxic to the gametes of the other, and
if this is so it is not impossible that we may some day be able to express
these differences in terms of Mendelian factors. The very nature of the
case makes it one of extreme difficulty for experimental investigation. At
any rate, we realise more clearly than before that the problem of species
is not one that can be resolved by the study of morphology or of
systematics. It is a problem in physiology.
* * * * *
{153}
CHAPTER XIV
ECONOMICAL
Since heredity lies at the basis of the breeder's work, it is evident that
any contribution to a more exact knowledge of this subject must prove of
service to him, and there is no doubt that he will be able to profit by
Mendelian knowledge in the conduct of his operations. Indeed, as we shall
see later, these ideas have already led to striking results in the raising
of new and more profitable varieties. In the first place, heredity is a
question of individuals. Identity of appearance is no sure guide to
reproductive qualities. Two individuals similarly bred and
indistinguishable in outward form may nevertheless behave entirely
differently when bred from. Take, for instance, the family of sweet peas
shown on Plate IV. The F_2 generation here consists of seven distinct
types, three sorts of purples, three sorts of reds, and whites. Let us
suppose that our object is to obtain a true breeding strain of the pale
purple picotee form. Now from the proportions in which they come we know
that the dilute colour is due to the absence of the factor which
intensifies the colour. Consequently the picotee cannot throw the {154} two
deeper shades of red or purple. But it may be heterozygous for the purpling
factor when it will throw the dilute red (tinged white), or it may be
heterozygous for either or both of the two colour factors (cf. p. 44), in
which case it will throw whites. Of the picotees which come in such a
family, therefore, some will give picotees, tinged whites, and whites,
others will give picotees and tinged whites only, others will give picotees
and whites only, while others, again, and these the least numerous, will
give nothing but picotees. The new variety is already fixed in a certain
definite proportion of the plants; in this particular instance in 1 out of
every 27. All that remains to be done is to pick out these plants. Since
all the picotees look alike, whatever their breeding capacity, the only way
to do this is to save the seed from a number of such plants _individually_,
and to raise a further generation. Some of them will be found to breed
true. The variety is then established, and may at once be put on the market
with full confidence that it will hereafter throw none of the other forms.
The all-important thing is to save and sow the seed of separate individuals
separately. However alike they look, the seed from different individuals
must on no account be mixed. Provided that due care is taken in this
respect no long and tedious process of selection is required for the
fixation of any given variety. Every possible variety arising from a cross
appears in the F_2 generation if only a sufficient {155} number is raised,
and of all these different varieties a certain proportion of each is
already fixed. Heredity is a question of individuals, and the recognition
of this will save the breeder much labour, and enable him to fix his
varieties in the shortest possible time.
Such cases as these of the sweet pea throw a fresh light upon another of
the breeder's conceptions, that of purity of type. Hitherto the criterion
of a "pure-bred" thing, whether plant or animal, has been its pedigree, and
the individual was regarded as more or less pure bred for a given quality
according as it could show a longer or shorter list of ancestors possessing
this quality. To-day we realise that this is not essential. The pure-bred
picotee appears in our F_2 family though its parent was a purple bicolor,
and its remoter ancestors whites for generations. So also from the cross
between pure strains of black and albino rabbits we may obtain in the F_2
generation animals of the wild agouti colour which breed as true to type as
the pure wild rabbit of irreproachable pedigree. The true test of the pure
breeding thing lies not in its ancestry but in the nature of the gametes
which have gone to its making. Whenever two similarly constituted gametes
unite, whatever the nature of the parents from which they arose, the
resulting individual is homozygous in all respects and must consequently
breed true. In deciding questions of purity it is to the gamete, and not to
ancestry, that our appeal must henceforth be made. {156}
Improvement is after all the keynote to the breeder's operations. He is
aiming at the production of a strain which shall combine the greatest
number of desirable properties with the least number of undesirable ones.
This good quality he must take from one strain, that from another, and that
again from a third, while at the same time avoiding all the poor qualities
that these different strains possess. It is evident that the Mendelian
conception of characters based upon definite factors which are transmitted
on a definite scheme must prove of the greatest service to him. For once
these factors have been determined, their distribution is brought under
control, and they can be associated together or dissociated at the
breeder's will. The chief labour involved is that necessary for the
determination of the factors upon which the various characters depend. For
it often happens that what appears to be a simple character turns out when
analysed to depend upon the simultaneous presence of several distinct
factors. Thus the Malay fowl breeds true to the walnut comb, as does also
the Leghorn to the single comb, and when pure strains are crossed all the
offspring have walnut combs. At first sight it would be not unnatural to
regard the difference as dependent upon the presence or absence of a single
factor. Yet, as we have already seen, two other types of comb, the pea and
the rose, make their appearance in the F_2 generation. Analysis shows that
the difference between the walnut {157} and the single is a difference of
two factors, and it is not until this has been determined that we can
proceed with certainty to transfer the walnut character to a single-combed
breed. Moreover, in his process of analysis the breeder must be prepared to
encounter the various phenomena that we have described under the headings
of interaction of factors, coupling and repulsion, and the recognition of
these phenomena will naturally influence his procedure. Or again, his
experiments may show him that one of the characters he wants, like the blue
of the Andalusian fowl, is dependent upon the heterozygous nature of the
individual which exhibits it, and if such is the case he will be wise to
refrain from any futile attempt at fixing it. If it is essential it must be
built up again in each generation, and he will recognise that the most
economical way of doing this is to cross the two pure strains so that all
the offspring may possess the desired character. The labour of analysis is
often an intricate and tedious business. But once done it is done once for
all. As soon as the various factors are determined, upon which the various
characters of the individual depend, as soon as the material to be made use
of has been properly analysed, the production and fixation of the required
combinations becomes a matter of simple detail.
An excellent example of the practical application of Mendelian principles
is afforded by the experiments which Professor Biffen has recently carried
out in Cambridge. {158} Taken as a whole English wheats compare favourably
with foreign ones in respect of their cropping power. On the other hand,
they have two serious defects. They are liable to suffer from the attacks
of the fungus which causes rust, and they do not bake into a good loaf.
This last property depends upon the amount of gluten present, and it is the
greater proportion of this which gives to the "hard" foreign wheat its
quality of causing the loaf to rise well when baked. For some time it was
held that "hard" wheat with a high glutinous content could not be grown in
the English climate, and undoubtedly most of the hard varieties imported
for trial deteriorated greatly in a very short time. Professor Biffen
managed to obtain a hard wheat which kept its qualities when grown in
England. But in spite of the superior quality of its grain from the baker's
point of view its cropping capacity was too low for it to be grown
profitably in competition with English wheats. Like the latter, it was also
subject to rust. Among the many varieties which Professor Biffen collected
and grew for observation he managed to find one which was completely immune
to the attacks of the rust fungus, though in other respects it had no
desirable quality to recommend it. Now as the result of an elaborate series
of investigations he was able to show that the qualities of heavy cropping
capacity, "hardness" of grain, and immunity to rust can all be expressed in
terms of Mendelian factors. Having once analysed his material {159} the
rest was comparatively simple, and in a few years he has been able to build
up a strain of wheat which combines the cropping capacity of the best
English varieties with the hardness of the foreign kinds, and at the same
time is completely immune to rust. This wheat has already been shown to
keep its qualities unchanged for several years, and there is little doubt
that when it comes to be grown in quantity it will exert an appreciable
influence on wheat-growing in Great Britain.
[Illustration: FIG. 30.
Curves to illustrate the influence of selection.]
It may be objected that it is often with small differences rather than with
the larger and more striking ones that the breeder is mainly concerned. It
does not matter much to him whether the colour of a pea flower is purple or
pink or white. But it does matter whether the plant bears rather larger
seeds than usual, or rather more of them. Even a small difference when
multiplied by the {160} size of the crop will effect a considerable
difference in the profit. It is the general experience of seedsmen and
others that differences of this nature are often capable of being developed
up to a certain point by a process of careful selection each generation. At
first sight this appears to be something very like the gradual accumulation
of minute variations through the continuous application of a selective
process. Some recent experiments by Professor Johannsen of Copenhagen set
the matter in a different light. One of his investigations deals with the
inheritance of the weight of beans, but as an account of these experiments
would involve us in the consideration of a large amount of detail we may
take a simple imaginary case to illustrate the nature of the conclusions at
which he arrived. If we weigh a number of seeds collected from a patch of
plants such as Johannsen's beans we should find that they varied
considerably in size. The majority would probably not diverge very greatly
from the general average, and as we approached the high or low extreme we
should find a constantly decreasing number of individuals with these
weights. Let us suppose that the weight of our seed varied between 4 and 20
grains, that the greatest number of seeds were of the mean weight, viz. 12
grains, and that as we passed to either extreme at 4 and 20 the number
became regularly less. The weight relation of such a collection of seeds
can be expressed by the accompanying curve (Fig. 30). Now if we select for
{161} sowing only that seed which weighs over 12 grains, we shall find that
in the next generation the average weight of the seed is raised and the
curve becomes somewhat shifted to the right as in the dotted line of Fig.
30. By continually selecting we can shift our curve a little more to the
right, _i.e._ we can increase the average weight of the seeds until at last
we come to a limit beyond which further selection has no effect. This
phenomenon has been long known, and it was customary to regard these
variations as of a continuous nature, _i.e._ as all chance fluctuations in
a homogeneous mass, and the effect of selection was supposed to afford
evidence that small continuous variations could be increased by this
process. But Johannsen's results point to another interpretation. Instead
of our material being homogeneous it is probably a mixture of several
strains each with its own average weight about {162} which the varying
conditions of the environment cause it to fluctuate. Each of these strains
is termed a PURE LINE. If we imagine that there are three such pure lines
in our imaginary case, with average weights 10, 12, 14 grains respectively,
and if the range of fluctuation of each of these pure lines is 12 grains,
then our curve must be represented as made up of the three components
A fluctuating between 4 and 16 with a mean of 10
B " " 6 " 18 " " 12
C " " 8 " 20 " " 14
[Illustration: FIG. 31.
Curves to illustrate the conception of pure lines in a population.]
as is shown in Fig. 31. A seed that weighs 12 grains may belong to any of
these three strains. It may be an average seed of B, or a rather large seed
of A, or a rather small seed of C. If it belongs to B its offspring will
average 12 grains, if to A they will average 10 grains, and if to C they
will average 14 grains. Seeds of similar weight may give a different result
because they happen to be fluctuations of different pure lines. But within
the pure line any seed, large or small, produces the average result for
that line. Thus a seed of line C which weighs 20 grains will give
practically the same result as one that weighs 10 grains.
On this view we can understand why selection of the largest seed raises the
average weight in the next generation. We are picking out more of C and
less of A and B, and as this process is repeated the proportion of C
gradually increases and we get the appearance of selection {163} acting on
a continuously varying homogeneous material and producing a permanent
effect. This is because the interval between the average weight of the
different pure lines is small compared with the environmental fluctuations.
None the less it is there, and the secret of separating and fixing any of
these pure lines is again to breed from the individual separately. As soon
as the pure line is separated further selection becomes superfluous.
Since the publication of Darwin's famous work upon the effects of cross and
self fertilisation, it has been generally accepted that the effect of a
cross is commonly, though not always, to introduce fresh vigour into the
offspring, though why this should be so we are quite at a loss to explain.
Continued close inbreeding, on the contrary, eventually leads to
deterioration, though, as in many self-fertilised plants, a considerable
number of generations may elapse before it shows itself in any marked
degree. The fine quality of many of the seedsman's choice varieties of
vegetables probably depends upon the fact that they had resulted from a
cross but a few generations back, and it is possible that they often oust
the older kinds not because they started as something intrinsically better,
but because the latter had gradually deteriorated through continuous
self-fertilisation. Most breeders are fully alive to the beneficial results
of a cross so far as vigour is concerned, but they often hesitate to embark
upon it owing to what was held {164} to be the inevitably lengthy and
laborious business of recovering the original variety and refixing it, even
if in the process it was not altogether lost. That danger Mendelism has
removed, and we now know that by working on these lines it is possible in
three or four generations to recover the original variety in a fixed state
with all the superadded vigour that follows from a cross.
Nor is the problem one that concerns self-fertilised plants only. Plants
that are reproduced asexually often appear to deteriorate after a few
generations unless a sexual generation is introduced. New varieties of
potato, for example, are frequently put upon the market, and their
excellent qualities give them a considerable vogue. Much is expected of
them, but time after time they deteriorate in a disappointing way and are
lost to sight. It is not improbable that we are here concerned with a case
in which the plants lose their vigour after a few asexual generations of
reproduction from tubers, and can only recover it with the stimulus that
results from the interpolation of a sexual generation. Unfortunately this
generally means that the variety is lost, for owing to the haphazard way in
which new kinds of potatoes are reproduced it is probable that most
cultivated varieties are complex heterozygotes. Were the potato plant
subjected to careful analysis and the various factors determined upon which
its variations depend, we should be in a position to remake continually any
good potato without {165} running the risk of losing it altogether, as is
now so often the case.
The application of Mendelian principles is likely to prove of more
immediate service for plants than animals, for owing to the large numbers
which can be rapidly raised from a single individual and the prevalence of
self-fertilisation, the process of analysis is greatly simplified. Even
apart from the circumstance that the two sexes may sometimes differ in
their powers of transmission, the mere fact of their separation renders the
analysis of their properties more difficult. And as the constitution of the
individual is determined by the nature and quality of its offspring, it is
not easy to obtain this knowledge where the offspring, as in most animals,
are relatively few. Still, as has been abundantly shown, the same
principles hold good here also, and there is no reason why the process of
analysis, though more troublesome, should not be effectively carried out.
At the same time, it affords the breeder a rational basis for some familiar
but puzzling phenomena. The fact, for instance, that certain characters
often "skip a generation" is simply the effect of dominance in F_1 and the
reappearance of the recessive character in the following generation.
"Reversion" and "atavism," again, are phenomena which are no longer
mysterious, but can be simply expressed in Mendelian terms as we have
already suggested in Chap. VI. The occasional appearance of a sport in a
supposedly pure strain is {166} often due to the reappearance of a
recessive character. Thus even in the most highly pedigreed strains of
polled cattle such as the Aberdeen Angus, occasional individuals with horns
appear. The polled character is dominant to the horned, and the occasional
reappearance of the horned animal is due to the fact that some of the
polled herd are heterozygous in this character. When two such individuals
are mated, the chances are 1 in 4 that the offspring will be horned. Though
the heterozygous individuals may be indistinguishable in appearance from
the pure dominant, they can be readily separated by the breeding test. For
when crossed by the recessive, in this case horned animals, the pure
dominant gives only polled beasts, while the heterozygous individual gives
equal numbers of polled and horned ones. In this particular instance it
would probably be impracticable to test all the cows by crossing with a
horned bull. For in each case it would be necessary to have several polled
calves from each before they could with reasonable certainty be regarded as
pure dominants. But to ensure that no horned calves should come, it is
enough to use a bull which is pure for that character. This can easily be
tested by crossing him with a dozen or so horned cows. If he gets no horned
calves out of these he may be regarded as a pure dominant and thenceforward
put to his own cows, whether horned or polled, with the certainty that all
his calves will be polled. {167}
Or, again, suppose that a breeder has a chestnut mare and wishes to make
certain of a bay foal from her. We know that bay is dominant to chestnut,
and that if a homozygous bay stallion is used a bay foal must result. In
his choice of a sire, therefore, the breeder must be guided by the previous
record of the animal, and select one that has never given anything but bays
when put to either bay or chestnut mares. In this way he will assure
himself of a bay foal from his chestnut mare, whereas if the record of the
sire shows that he has given chestnuts he will be heterozygous, and the
chances of his getting a bay or a chestnut out of a chestnut mare are
equal.
It is not impossible that the breeder may be unwilling to test his animals
by crossing them with a different breed through fear that their purity may
be thereby impaired, and that the influence of the previous cross may show
itself in succeeding generations. He might hesitate, for instance, to test
his polled cows by crossing them with a horned bull for fear of getting
horned calves when the cows were afterwards put to a polled bull of their
own breed. The belief in the power of a sire to influence subsequent
generations, or telegony as it is sometimes called, is not uncommon even
to-day. Nevertheless, carefully conducted experiments by more than one
competent observer have failed to elicit a single shred of unequivocal
evidence in favour of the view. Until we have evidence based upon
experiments which are capable of {168} repetition, we may safely ignore
telegony as a factor in heredity.
Heterozygous forms play a greater part in the breeding of animals than of
plants, for many of the qualities sought after by the breeder are of this
nature. Such is the blue of the Andalusian fowl, and, according to
Professor Wilson, the roan of the Shorthorn is similar, being the
heterozygous form produced by mating red with white. The characters of
certain breeds of canaries and pigeons again appear to depend upon their
heterozygous nature. Such forms cannot, of course, ever be bred true, and
where several factors are concerned they may when bred together produce but
a small proportion of offspring like themselves. As soon, however, as their
constitution has been analysed and expressed in terms of Mendelian factors,
pure strains can be built up which when crossed will give nothing but
offspring of the desired heterozygous form.
The points with which the breeder is concerned are often fine ones, not
very evident except to the practised eye. Between an ordinary Dutch rabbit
and a winner, or between the comb of a Hamburgh that is fit to show and one
that is not, the differences are not very apparent to the uninitiated.
Whether Mendelism will assist the breeder in the production of these finer
points is at present doubtful. It may be that these small differences are
heritable, such as those that form the basis of Johannsen's pure lines. In
this case the breeder's outlook is {169} hopeful. But it may be that the
variations which he seeks to perpetuate are of the nature of fluctuations,
dependent upon the earlier life conditions of the individual, and not upon
the constitution of the gametes by which it was formed. If such is the
case, he will get no help from the science of heredity, for we know of no
evidence which might lead us to suppose that variations of this sort can
ever become fixed and heritable.
* * * * *
{170}
CHAPTER XV
MAN
[Illustration: FIG. 32.
Normal and brachydactylous hands placed together for comparison. (From
Drinkwater.)]
[Illustration: FIG. 33.
Radiograph of a brachydactylous hand.]
Though the interest attaching to heredity in man is more widespread than in
other animals, it is far more difficult to obtain evidence that is both
complete and accurate. The species is one in which the differentiating
characters separating individual from individual are very numerous, while
the number of the offspring is comparatively few, and the generations are
far between. For these reasons, even if it were possible, direct
experimental work with man would be likely to prove both tedious and
expensive. There is, however, another method besides the direct one from
which something can be learned. This consists in collecting all the
evidence possible, arranging it in the form of pedigrees, and comparing it
with standard cases already worked out in animals and plants. In this way
it has been possible to demonstrate in man the existence of several
characters showing simple Mendelian inheritance. As few besides medical men
have hitherto been concerned practically with heredity, such records as
exist are, for the most part, records of deformity or of disease. So it
happens that most of the {171} pedigrees at present available deal with
characters which are usually classed as abnormal. In some of these the
inheritance is clearly Mendelian. One of the cases which has been most
fully worked out is that of a deformity known as brachydactyly. In
brachydactylous people the {172} whole of the body is much stunted, and the
fingers and toes appear to have two joints only instead of three (cf. Figs.
32 and 33). The inheritance of this peculiarity has been carefully
investigated by Dr. Drinkwater, who collected all the data he was able to
find among the members of a large family in which it occurred. The result
is the pedigree shown on p. 173. It is assumed that all who are recorded as
having offspring were married to normals. Examination of the pedigree
brings out the facts (1) that all affected individuals have an affected
parent; (2) that none of the unaffected individuals, though sprung from the
affected, ever have descendants who are affected, and (3) that in families
where both affected and unaffected {173} occur, the numbers of the two
classes are, on the average, equal. (The sum of such families in the
complete pedigree is thirty-nine affected and thirty-six normals.) It is
obvious that these are the conditions which are fulfilled in a simple
Mendelian case, and there is nothing in this pedigree to contradict the
assertion that brachydactyly, whatever it may be due to, behaves as a
simple dominant to the normal form, _i.e._ that it depends upon a factor
which the normal does not contain. The recessive normals cannot transmit
the affected condition whatever their ancestry. Once free they are always
free, and can marry other normals with full confidence that none of their
children will show the deformity.
[Illustration: FIG. 34.
Pedigree of Drinkwater's brachydactylous family. The affected members are
indicated by black and the normals by light circles.]
{174}
The evidence available from pedigrees has revealed the simplest form of
Mendelian inheritance in several human defects and diseases, among which
may be mentioned presenile cataract of the eyes, an abnormal form of skin
thickening in the palms of the hands and soles of the feet, known as
tylosis, and epidermolysis bullosa, a disease in which the skin rises up
into numerous bursting blisters.
Among the most interesting of all human pedigrees is one recently built up
by Mr. Nettleship from the records of a night-blind family living near
Monpelier in the south of France. In night-blind people the retina is
insensitive to light which falls below a certain intensity, and such people
are consequently blind in failing daylight or in moonlight. As the
Monpelier case had excited interest for some time, the records are
unusually complete. They commence with a certain Jean Nougaret, who was
born in 1637, and suffered from night-blindness, and they end for the
present with children who are to-day but a few years of age. Particulars
are known of over 2000 of the descendants of Jean Nougaret. Through ten
generations and nearly three centuries the affection has behaved as a
Mendelian dominant, and there is no sign that long-continued marriage with
folk of normal vision has produced any amelioration of the night-blind
state. {175}
[Illustration: FIG. 35.
Pedigree of a hæmophilic family. Affected (all males) represented by black,
and normals of both sexes by light circles. (From Stahel.)]
Besides cases such as these where a simple form of Mendelian inheritance is
obviously indicated, there are others which are more difficult to read. Of
some it may be said that on the whole the peculiarity behaves as though it
were an ordinary dominant; but that exceptions occur in which affected
children are born to unaffected parents. It is not impossible that the
condition may, like colour in the sweet pea, depend upon the presence or
absence of more than one factor. In none of these cases, however, are the
data sufficient for determining with certainty whether this is so or not.
A group of cases of exceptional interest is that in which the incidence of
disease is largely, if not absolutely, restricted to one sex, and so far as
is hitherto known the burden is invariably borne by the male. In the
inheritance of colour-blindness (p. 117) we have already discussed an
instance in which the defect is rare, though not {176} unknown, in the
female. Sex-limited inheritance of a similar nature is known for one or two
ocular defects, and for several diseases of the nervous system. In the
peculiarly male disease known as hæmophilia the blood refuses to clot when
shed, and there is nothing to prevent great loss from even a superficial
scratch. In its general trend the inheritance of hæmophilia is not unlike
that of horns among sheep, and it is possible that we are here again
dealing with a character which is dominant in one sex and recessive in the
other. But the evidence so far collected points to a difference somewhere,
for in hæmophilic families the affected males, instead of being equal in
number to the unaffected, show a considerable preponderance. The
unfortunate nature of the defect, however, forces us to rely for our
interpretation almost entirely upon the families produced by the unaffected
females who can transmit it. Our knowledge of the offspring of "bleeding"
males is as yet far too scanty, and until it is improved, or until we can
find some parallel case in animals or plants, the precise scheme of
inheritance for hæmophilia must remain undecided.
Though by far the greater part of the human evidence relates to abnormal or
diseased conditions, a start has been made in obtaining pedigrees of normal
characters. From the ease with which it can be observed, it was natural
that eye-colour should be early selected as a subject of investigation, and
the work of Hurst and others {177} has clearly demonstrated the existence
of one Mendelian factor in operation here. Eyes are of many colours, and
the colour depends upon the pigment in the iris. Some eyes have pigment on
both sides of the iris--on the side that faces the retina as well as on the
side that looks out upon the world. Other eyes have pigment on the retinal
side only. To this class belong the blues and clear greys; while the eyes
with pigment in front of the iris also are brown, hazel, or green in
various shades according to the amount of pigment present. In albino
animals the pigment is entirely absent, and as the little blood-vessels are
not obscured the iris takes on its characteristic pinkish-red appearance.
The condition in which pigment is present in front of the iris is dominant
to that in which it is absent. Greens, browns, or hazels mated together
may, if heterozygous, give the recessive blue, but no individuals of the
brown class are to be looked for among the offspring of blues mated
together. The blues, however, may carry factors which are capable of
modifying the brown. Just as the pale pink-tinged sweet pea (Pl. IV., 9)
when mated with a suitable white gives only deep purples, so an eye with
very little brown pigment mated with certain blues produces progeny of a
deep brown, far darker than either parent. The blue may carry a factor
which brings about intensification of the brown pigment. There are
doubtless other factors which modify the brown when present, but we do not
yet know enough of the {178} inheritance of the various shades to justify
any statement other than that the heredity of the pigment in front of the
iris behaves as though it were due to a Mendelian factor.
Even this fact is of considerable importance, for it at once suggests that
the present systems of classification of eye-colours, to which some
anthropologists attach considerable weight, are founded on a purely
empirical and unsatisfactory basis. Intensity of colour is the criterion at
present in vogue, and it is customary to arrange the eye-colours in a scale
of increasing depth of shade, starting with pale greys and ending with the
deepest browns. On this system the lighter greens are placed among the
blues. But we now know that blues may differ from the deep browns in the
absence of only a single factor, while, on the other hand, the difference
between a blue and a green may be a difference dependent upon more than one
factor. To what extent eye-colour may be valuable as a criterion of race it
is at present impossible to say, but if it is ever to become so, it will
only be after a searching Mendelian analysis has disclosed the factors upon
which the numerous varieties depend.
A discussion of eye-colour suggests reflections of another kind. It is
difficult to believe that the markedly different states of pigmentation
which occur in the same species are not associated with deep-seated
chemical differences influencing the character and bent of the individual.
{179} May not these differences in pigmentation be coupled with and so
become in some measure a guide to mental and temperamental characteristics?
In the National Portrait Gallery in London the pictures of celebrated men
and women are largely grouped according to the vocations in which they have
succeeded. The observant will probably have noticed that there is a
tendency for a given type of eye-colour to predominate in some of the
larger groups. It is rare to find anything but a blue among the soldiers
and sailors, while among the actors, preachers, and orators the dark eye is
predominant, although for the population as a whole it is far scarcer than
the light. The facts are suggestive, and it is not impossible that future
research may reveal an intimate connection between peculiarities of
pigmentation and peculiarities of mind.
The inheritance of mental characters is often elusive, for it is frequently
difficult to appraise the effects of early environment in determining a
man's bent. That ability can be transmitted there is no doubt, for this is
borne out by general experience, as well as by the numerous cases of able
families brought together by Galton and others. But when we come to inquire
more precisely what it is that is transmitted we are baffled. A
distinguished son follows in the footsteps of a distinguished father. Is
this due to the inheritance of a particular mental aptitude, or is it an
instance of general mental ability displayed in a field rendered attractive
by early association? We have {180} at present very little definite
evidence for supposing that what appear to be special forms of ability may
be due to specific factors. Hurst, indeed, has brought forward some facts
which suggest that musical sense sometimes behaves as a recessive
character, and it is likely that the study of some clean-cut faculty such
as the mathematical one would yield interesting results.
The analysis of mental characters will no doubt be very difficult, and
possibly the best line of attack is to search for cases where they are
associated with some physical feature such as pigmentation. If an
association of this kind be found, and the pigmentation factors be
determined, it is evident that we should thereby obtain an insight into the
nature of the units upon which mental conditions depend. Nor must it be
forgotten that mental qualities, such as quickness, generosity,
instability, etc.,--qualities which we are accustomed to regard as
convenient units in classifying the different minds with which we are daily
brought into contact,--are not necessarily qualities that correspond to
heritable units. Effective mental ability is largely a matter of
temperament, and this in turn is quite possibly dependent upon the various
secretions produced by the different tissues of the body. Similar nervous
systems associated with different livers might conceivably result in
individuals upon whose mental ability the world would pass a very different
judgment. Indeed, it is not at all impossible {181} that a particular form
of mental ability may depend for its manifestation, not so much upon an
essential difference in the structure of the nervous system, as upon the
production by another tissue of some specific poison which causes the
nervous system to react in a definite way. We have mentioned these
possibilities merely to indicate how complex the problem may turn out to
be. Though there is no doubt that mental ability is inherited, what it is
that is transmitted, whether factors involving the quality and structure of
the nervous system itself, or factors involving the production of specific
poisons by other tissues, or both together, is at present uncertain.
Little as is known to-day of heredity in man, that little is of
extraordinary significance. The qualities of men and women, physical and
mental, depend primarily upon the inherent properties of the gametes which
went to their making. Within limits these qualities are elastic, and can be
modified to a greater or lesser extent by influences brought to bear upon
the growing zygote, provided always that the necessary basis is present
upon which these influences can work. If the mathematical faculty has been
carried in by the gamete, the education of the zygote will enable him to
make the most of it. But if the basis is not there, no amount of education
can transform that zygote into a mathematician. This is a matter of common
experience. Neither is there any reason for supposing that the superior
education of a {182} mathematical zygote will thereby increase the
mathematical propensities of the gametes which live within him. For the
gamete recks little of quaternions. It is true that there is progress of a
kind in the world, and that this progress is largely due to improvements in
education and hygiene. The people of to-day are better fitted to cope with
their material surroundings than were the people of even a few thousand
years ago. And as time goes on they are able more and more to control the
workings of the world around them. But there is no reason for supposing
that this is because the effects of education are inherited. Man stores
knowledge as a bee stores honey or a squirrel stores nuts. With man,
however, the hoard is of a more lasting nature. Each generation in using it
sifts, adds, and rejects, and passes it on to the next a little better and
a little fuller. When we speak of progress we generally mean that the hoard
has been improved, and is of more service to man in his attempts to control
his surroundings. Sometimes this hoarded knowledge is spoken of as the
inheritance which a generation receives from those who have gone before.
This is misleading. The handing on of such knowledge has nothing more to do
with heredity in the biological sense than has the handing on from parent
to offspring of a picture, or a title, or a pair of boots. All these things
are but the transfer from zygote to zygote of something extrinsic to the
species. Heredity, on the other hand, deals with the {183} transmission of
something intrinsic from gamete to zygote and from zygote to gamete. It is
the participation of the gamete in the process that is our criterion of
what is and what is not heredity.
Better hygiene and better education, then, are good for the zygote, because
they help him to make the fullest use of his inherent qualities. But the
qualities themselves remain unchanged in so far as the gamete is concerned,
since the gamete pays no heed to the intellectual development of the zygote
in whom he happens to dwell. Nevertheless, upon the gamete depend those
inherent faculties which enable the zygote to profit by his opportunities,
and, unless the zygote has received them from the gamete, the advantages of
education are of little worth. If we are bent upon producing a permanent
betterment that shall be independent of external circumstances, if we wish
the national stock to become inherently more vigorous in mind and body,
more free from congenital physical defect and feeble mentality, better able
to assimilate and act upon the stores of knowledge which have been
accumulated through the centuries, then it is the gamete that we must
consult. The saving grace is with the gamete, and with the gamete alone.
People generally look upon the human species as having two kinds of
individuals, males and females, and it is for them that the sociologists
and legislators frame their schemes. This, however, is but an imperfect
view to {184} take of ourselves. In reality we are of four kinds, male
zygotes and female zygotes, large gametes and small gametes, and heredity
is the link that binds us together. If our lives were like those of the
starfish or the sea-urchin, we should probably have realised this sooner.
For the gametes of these animals live freely, and contract their marriages
in the waters of the sea. With us it is different, because half of us must
live within the other half or perish. Parasites upon the rest, levying a
daily toll of nutriment upon their hosts, they are yet in some measure the
arbiters of the destiny of those within whom they dwell. At the moment of
union of two gametes is decided the character of another zygote, as well as
the nature of the population of gametes which must make its home within
him. The union once affected the inevitable sequence takes its course, and
whether it be good, or whether it be evil, we, the zygotes, have no longer
power to alter it. We are in the hands of the gamete; yet not entirely. For
though we cannot influence their behaviour we can nevertheless control
their unions if we choose to do so. By regulating their marriages, by
encouraging the desirable to come together, and by keeping the undesirable
apart we could go far towards ridding the world of the squalor and the
misery that come through disease and weakness and vice. But before we can
be prepared to act, except, perhaps, in the simplest cases, we must learn
far more about them. At present we are woefully ignorant {185} of much,
though we do know that full knowledge is largely a matter of time and
means. One day we shall have it, and the day may be nearer than most
suspect. Whether we make use of it will depend in great measure upon
whether we are prepared to recognise facts, and to modify or even destroy
some of the conventions which we have become accustomed to regard as the
foundations of our social life. Whatever be the outcome, there can be
little doubt that the future of our civilisation, perhaps even the
possibility of a future at all, is wrapped up with the recognition we
accord to those who live unseen and inarticulate within us--the fateful
race of gametes so irrevocably bound to us by that closest of all ties,
heredity.
* * * * * {187}
APPENDIX
As some readers may possibly care to repeat Mendel's experiments for
themselves, a few words on the methods used in crossing may not be
superfluous. The flower of the pea with its standard, wings, and median
keel is too familiar to need description. Like most flowers it is
hermaphrodite. Both male and female organs occur on the same flower, and
are covered by the keel. The anthers, ten in number, are arranged in a
circle round the pistil. As soon as they are ripe they burst and shed their
pollen on the style. The pollen tubes then penetrate the stigma, pass down
the style, and eventually reach the ovules in the lower part of the pistil.
Fertilisation occurs here. Each ovule, which is reached by a pollen tube,
swells up and becomes a seed. At the same time the fused carpels enclosing
the ovules enlarge to form the pod. When this, the normal mode of
fertilisation, takes place, the flower is said to be SELFED.
In crossing, it is necessary to emasculate a flower on the plant chosen to
be the female parent. For this purpose a young flower must be taken in
which the anthers have not yet burst. The keel is depressed, and the
stamens bearing the anthers are removed at their base by a {188} pair of
fine forceps. It will probably be found necessary to tear the keel slightly
in order to do this. The pistil is then covered up again with the keel, and
the flower is enclosed in a bag of waxed paper until the following day. The
stigma is then again exposed and dusted with ripe pollen from a flower of
the plant selected as the male parent. This done, the keel is replaced, and
the flower again enclosed in its bag to protect it from the possible
attentions of insects until it has set seed. The bag may be removed in
about a week after fertilisation. It is perhaps hardly necessary to add
that strict biological cleanliness must be exercised during the fertilising
operations. This is readily attained by sterilising fingers and forceps
with a little strong spirit before each operation, thereby ensuring the
death of any foreign pollen grains which may be present.
The above method applies also to sweet peas, with these slight
modifications. As the anthers ripen relatively sooner in this species,
emasculation must be performed at a rather earlier stage. It is generally
safe to choose a bud about three parts grown. The interval between
emasculation and fertilisation must be rather longer. Two to three days is
generally sufficient. Further, the sweet pea is visited by the leaf-cutter
bee, _Megachile_, which, unlike the honey bee, is able to depress the keel
and gather pollen. If the presence of this insect is suspected, it is
desirable to guard against the risk of admixture of {189} foreign pollen by
selecting for pollinating purposes a flower which has not quite opened. If
the standard is not erected, it is unlikely to have been visited by
_Megachile_. Lastly, it not infrequently happens that the little beetle
_Meligethes_ is found inside the keel. Such flowers should be rejected for
crossing purposes.
* * * * *
{191}
INDEX
_Abraxas grossulariata_, 99
"Acquired" characters, 14
Adaptation, 143
Agouti mice, 50
Albino mice, 50
Albinos, nature of, 53
_Amauris_, 144
Analysis of types, 156
Ancestral Heredity, Law of, 13
Andalusian fowls, 70
Axil colour in sweet peas, 93
Bateson, W., 14, 29, 55, 116, 132, 141
Biffen, R. H., 157
Blue Andalusian fowls, 71
Brachydactyly, 171
Bryony, 120
Bush sweet peas, 63
Castle, 132
Cattle, horns in, 86, 166
Colour, nature of, in flowers, 48
Colour-blindness, 117
Combs of fowls, 33, 43
Correns, C., 29, 120
Coupling of characters in gametes, 93
Cuénot, 50, 119
"Cupid" sweet peas, 62
Currant moth, 99
Darwin, C., 10, 65, 147, 163
De Vries, H., 15, 29, 141
Discontinuity in variation, 14
Dominant characters, 18
Doncaster, L., 99
Drinkwater, H., 172
Dutch rabbits, 60
Eggs, 2
Environment, influence of, 137
_Euralia_, 144
Evolution, 10, 85, 139
Eye, in primulas, 55
Eye-colour, in man, 176
Factor, definition of, 31
Factors, interaction of, 42
Fertilisation, 3
Fertilisation, self- and cross-, 163
Fixation of varieties, 153
Fluctuations, 138
Fowls, coloured from whites, 49, 73
Galton, 13, 179
Gametes, nature of, 6
Gregory, R. P., 55, 93
Hæmophilia, 176
Hardy, G. H., 147
Heterozygote, definition of, 28
Heterozygote, of intermediate form, 68
_Hieracium_, 27, 132
Himalayan rabbits, 60
Homostyle primulas, 56
Homozygote, definition of, 28
Hooded sweet peas, 89
Horses, bay and chestnut in, 167
Hurst, C. C., 62, 176, 180
Immunity in wheat, 158
Individuality, 135
Inhibition, factors for, 74, 108
Intermediates, 125
{192}
Johannsen, W., 160
Lop-eared rabbits, 132
Mendel, 8, 17, 26, 132
Mental characters, 180
Mice, inheritance of coat colour in, 50
Mimicry, 143
_Mirabilis_, 151
Morgan, T. H., 116
Mulattos, 129
Mutation, 83, 138
Nägeli, C., 26
Natural selection, 11, 140, 142, 149
Nettleship, E., 175
Night-blindness, 175
_Pararge egeria_, 132
Parkinson, J., 122
Pea comb, 33
Peas, coloured flowers in, 24
Peas, tall and dwarf, 18
Pigeons, 86
Pin-eye in primulas, 55
_Pisum_, 17
Primulas, 31, 55, 68, 93
Pollen, 3
Pollen of sweet peas, 92
Pomace fly, 115
Population, inheritance of characters in a, 147
Presence and Absence theory, 35
Pure lines, 162
Purity of gametes, 24
Purity of type, 155
Rabbits, 53, 60
Ratios, Mendelian--
3 : 1, 20
9 : 3 : 3 : 1, 25, 34
9 : 3 : 4, 51
9 : 7, 49
Ray, John, 143
Recessive characters, 19
Repulsion between factors, 90
Reversion, 59, 165
in rabbits, 59
in sweet peas, 62
in fowls, 65
in pigeons, 65
Rose comb, 33
Saunders, E. R., 54, 122
Seeds, nature of, 4
Segregation, 22
Selection, 162
Sheep, horns in, 76
Silky fowls, 30, 105
Single comb, 32
Species, nature of, 150
Species, origin of, 11
Speckled wood butterfly, 132
Spermatozoa, 3
Sports, 147
Staples-Browne, R., 66
Sterility, 151
Sterility in sweet peas, 93
Stocks, double, 122
Stocks, hoariness in, 54
Sweet pea, colour in, 44, 79
history of, 82
inheritance of hood in, 89
inheritance of size in, 62
Telegony, 167
Thrum-eye in primulas, 55
Toe, extra toe in poultry, 76
Tschermak, E., 29
Unit-character, definition of, 31
Variation, 14, 137, 139
Walnut comb, 33
Weismann, A., 13
Wheat, beard in, 74
experiments with, 157
White, dominant in poultry, 72
Wilson, J., 168
Yellow mice, 119
Zygotes, nature of, 5
* * * * *
Notes
* * * * *
[1] Cf. note on p. 171.
[2] It has been found convenient to denote the various generations
resulting from a cross by the signs F_1, F_2, F_3, etc. F_1 on this system
denotes the first filial generation, F_2 the second filial generation
produced by two parents belonging to the F_1 generation, and so on.
[3] Hurst's original cross was between a Belgian hare and an albina Angora,
which turned out to be a masked Dutch.
[4] The Spot is an almost white bird, the colour being confined to the tail
and the characteristic spot on the head.
[5] The reader who searches florists' catalogues for these varieties will
probably experience disappointment. The sweet pea has been much "improved"
in the past few years, and it is unlikely that the modern seedsman would
list such unfashionable forms.
[6] It is to be understood that wherever a given factor is present the
plant may be homozygous or heterozygous for it without alteration in its
colour.
[7] It should be mentioned that as the shape of the pollen coat, like that
of the seed coat, is a maternal character, all the grains of any given
plant are either long or else round. The two kinds do not occur together on
the same plant.
[8] For the most recent discussion of this peculiar case the reader is
referred to Professor Castle's paper in _Science_, December 16, 1910.
[9] _Paradisus Terrestris_, London, 1629, p. 261.
* * * * *
Transcriber's note:
Corrections made to printed original.
Page 36, "two sorts, RP and Rp": 'PR and Rp' in original.
Page 51, "9 contain both C and G": 'c and G' in original.
Page 184, "in the simplest cases": 'simples' in original.
Footnote 3, "turned out to be a masked Dutch": 'turned to
out be' in original.
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