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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_ x _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 x Brown Leghorn
Cock 105
20. Scheme of Inheritance of Brown Leghorn Hen x Silky
Cock 106
21. Scheme of F_1 (ex Brown Leghorn x Silky Cock) crossed
with pure Brown Leghorn 107
22. Scheme for Silky Hen x Brown Leghorn Cock 108
23. Scheme for Brown Leghorn Hen x Silky Cock 109
24. Diagram illustrating Nature of Offspring from Brown Leghorn
Hen x 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 x 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 Haemophilic 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}