Genetics; an introduction to the study of heredity

Front matter

GENET 1 1

ANlMTRODUCTION'ljjiiP r H E S TV D Y 0 f ^ H E RE D I T Y

:|H|i!!l!i| Iji

Preface

The following pages had their origin in a course of lectures upon Heredity, given at Brown University during the winter of 1911-1912, which were amplified and repeated in part the following summer at Cold Spring Harbor, Long Island, before the biological summer school of the Brooklyn Institute of Arts and Sciences.

An attempt has been made to summarize for the intelligent, but uninitiated, reader some of the more recent phases of the questions of heredity which are at present agitating the biological world. It is hoped that this summary will not only be of interest to the general reader, but that it will also be of service in college courses dealing with evolution and heredity.

The subject of heredity concerns every one, but many of those who wish to become better informed regarding it are either too busily engaged or lack the opportunity to study the matter out for themselves. The recent literature in this field is already very large, with every indication that much more is about to follow, which is a further discouragement to nontechnical readers.

It may not be a thankless task, therefore, out of the jargon of many tongues to raise a single voice

vii viii PREFACE

which shall attempt to tell the tale of heredity. There may be a certain advantage in having as spokesman one who is not at present immersed in the arduous technical investigations that are making the tale worth telling. The difficulties in understanding this complicated subject may possibly be realized better by one who is himself still struggling with them, than by the seasoned expert who has long since forgotten that such difficulties exist.

Among others I am particularly indebted to Dr. C. B. Davenport for many helpful suggestions, to my colleague, Professor A. D. Mead, for reading the manuscript critically, to Dr. S. I. Kornhauser who gave valuable aid in connection with the chapter on the Determination of Sex, and to my wife for assistance in final preparation for the press.

I wish to thank Professor H. S. Jennings and Dr. H. H. Goddard, who have given generous permission to copy certain diagrams, as well as The Outlook Company and The Macmillan Company for the use of figures 24 and 66, respectively.

The fact that all the suggestions which were at various times offered by my kindly critics have not been incorporated in the text, absolves them from responsibility for whatever remains.

H. E. W.

Providence, R. I., September, 1912.

CHAPTER

I. Introduction.

1. The triangle of life ' .

2. A definition of heredity

3. The maintenance of life

4. Somatoplasm and germplasm

II. The Carriers of the Heritage.

2. The cell theory .

3. A typical cell

4. Mitosis ....

5. Amitosis ....

6. Sexual reproduction .

7. Maturation

8. Fertilization

9. Parthenogenesis

10. The hereditary bridge

11. The determiners of heredity

12. The chromosome theory

13. The enzyme theory of heredity

III. Variation.

1. The most invariable thing in nature

2. The universality of variation

3. The kinds of variation with respect to

a. Nature

h. Duplication

c. Utility

d. Direction in evolution

e. Source /. Normality g. Degree of continuity . h. Character . i. Relation to an average standard j. Heritability . . . .

their

PAGE 1

14 14 15

IX

CHAPTER PAGE

4. Methods of studying variation 42

5. Biometry 42

6. Fluctuating variation 43

7. The interpretation of variation curves ... 47

a. Relative variability ...... 47

b. Bimodal curves ...... 47

c. Skew polygons ....... 50

8. Graduated and integral variations .... 52

9. The causes of variation ...... 52

a. Darwin's attitude ...... 52

b. Lamarck's attitude ...... 53

c. Weismann's attitude . .... 55

d. Bateson's attitude ...... 55

IV. Mutation.

1. The mutation theory ....... 56

2. Mutation and fluctuation ...... 57

3. Freaks 58

4. Kinds of mutation ....... 59

5. Species and varieties ....... 60

6. Plant mutations found in nature .... 63

7. Lamarck's evening primrose ..... 64

8. Some mutations among animals .... 67

9. Possible explanations of mutation .... 69 10. A summary of the mutation theory .... 72

V. The Inheritance of Acquired Characters.

1. Summary of preceding chapters ..... 74

2. The bearing of this chapter upon genetics ... 75

3. The importance of the question ..... 75

4. An historical sketch of opinion ..... 76

5. Confusion in definitions ...... 77

6. Weismann's conception of acquired characters . . 78

7. The distinction between germinal and somatic charac-

ters 79

8. What variations reappear ? . . . . .80

9. What may cause germplasm to vary or to acquire

new characters ? . . . . . .81

10. Weismann's reasons for doubting the inheritance of

acquired characters ..... 84

XI

CHAPTER

11. No known mechanism for impressing germplasm with

somatic characters . . . . .

12. Evidence for the inheritance of acquired characters

inconclusive .

a. Mutilations

b. Environmental effects

c. The effects of use or disuse

d. Disease transmission

13. The germplasm theory sufficient to

facts of heredity .

14. The opposition to Weismann .

account for the

PAGE

86 87 88 91 92

94 95 96

VI. The Pure Line.

1. The unit character method of attack

2. Galton's law of regression

3. The idea of the pure line

4. Johanssen's nineteen beans

5. Cases similar to Johanssen's pure lines

6. Tower's potato-beetles

7. Jennings' work on Paramecium

8. Phenotypical and genotypical distinctions

9. The distinction between a population and a pure line 10. Pure lines and natural selection . . . .

VII. Segregation and Dominance.

1. Methods of studying heredity .

2. The melting-pot of cross-breeding

a. Blending inheritance

b. Alternative inheritance .

c. Particulate inheritance

3. Johann Gregor Mendel

4. Mendel's experiments on garden peas

5. Some further instances of Mendel's law

6. The principle of segregation

7. Homozygotes and heterozygotes

8. The identification of a heterozygote .

9. The presence and absence hypothesis 10. Dihybrids

Xll

CHAPTER PAGE

11. The case of the trihybrid 140

13. Summary 144 ones new

Vin. Reversion to Old Types and the Making of New

Ones.

1. The distinction between reversion and atavism

2. False reversion .....

a. Arrested development

6. Vestigial structures ....

c. Acquired characters resembling ancestral

d. Convergent variation

e. Regression .....

3. Explanation of reversion ....

4. Some methods of improving old and establishing

types

The method of Hallet The method of Rimpau . .

The method of de Vries .

The method of Vilmorin .

The method of Johanssen The method of Burbank The method of Mendel .

5. The factor hypothesis ....

a. Bateson's sweet peas

Castle's agouti guinea-pigs

Cuenot's spotted mice

Miss Durham's intensified mice

Castle's brown-eyed, yellow guinea-pigs

6. Rabbit phenotypes .....

7. The kinds of gray rabbits a. b. c. d. e.

/. 9- b. c. d. e.

IX. Blending Inheritance.

1. The relative value of dominance and segregation . 174

2. Imperfect dominance ...... 175

3. Delayed dominance ...... 177

4. " Reversed " dominance 178

Xlll

CHAPTER

5. Potency .

a. Total potency

b. Partial potency

c. Failure of potency

7. The case of rabbit ears

8. The Nilsson-Ehle discovery

9. The application of Nilsson-Ehle's explanation to the

case of rabbit ear-length 10. Human skin color .

X. The Determination of Sex.

1. Speculations, ancient and modern

2. The nutrition theory

3. The statistical study of sex

4. Monochorial twins .

5. Selectiv^e fertilization

6. The neo-Mendelian theory of sex

a. Microscopical evidence

1. The "x" chromosome

2. Various forms of x chromosomes

3. Sex chromosomes in parthenogenesis

b. Castration and regeneration experiments

c. Sex-limited inheritance

1. Color-blindness

2. The English currant-moth

d. Behavior of hermaphrodites in heredity

XI. The Application to Man.

1. 2. 3.

PAGE

193 196

5. 6.

The application of genetics to man .... 224

Modifying factors in the case of man . . . 225

Experiments in human heredity .... 227

a. The Jukes 227

6. The descendants of Jonathan Edwards . . 228

c. The Kallikak family 229

Moral and mental characters behave like physical

ones 230

The character of human traits ..... 231

Hereditarv defects . . . . . . . 232

XIV

CHAPTEB

7. The control of defects ....

8. Inbreeding

9. Experiments to test the effects of inbreeding

10. The influence of proximity

11. Inbreeding in the Ught of Mendelism

XII. Human Conservation.

1. How mankind may be improved

2. More facts needed ......

3. More appHcation of what we know necessary .

4. The restriction of undesirable germplasm through

a. The control of immigration

More discriminating marriage laws An educated sentiment .

The segregation of defectives .

Drastic measures The conservation of desirable germplasm By subsidizing the fit By enlarging individual opportunity By preventing germinal waste .

1. Preventable death .

2. Social hindrances 6. Who shall sit in judgment ?

h. c. d. e.

a. b. c.

XIII, Bibliography

XIV. Index

PAGE

235 238 240 241 242

244 245 247

Chapter I

INTRODUCTION 1. The Triangle of Life

Within a generation the center of biological interest has gradually been swinging from the origin of species to the origin of the individual. The nineteenth century was Darwin's century. His monumental work "On the Origin of Species by Means of Natural Selection," which appeared in 1859, not only dominated the biological sciences but also influenced profoundly many other realms of thought, particularly those of philosophy and theology.

Now, at the beginning of the twentieth century, a particular emphasis is being laid upon the study of heredity. The interpretation of investigations along this line of research has been made possible through the cumulative discoveries of many things that were not known in Darwin's day. Trained students have been patiently and persistently bending over improved microscopes, untangling the mysteries of the cell, while an increasing host of investigators, inspired by the Austrian monk Mendel, have been industriously devoting their energies to nOF&RTY UBRARY

n, C. State C»Be|i breeding animals and plants with an insight denied to breeders of preceding centuries.

The study of the origin of the individual, which has grown out of the more general consideration of the origin of species, forms the subject-matter of heredity, or, to use the more definitive word of Bateson, of genetics.

It is not with the individual as a whole that

HER! T A G E

Fig. 1. — The triangle of life.

genetics is chiefly concerned, but rather with characteristics of the individual.

Three factors determine the characteristics of an individual, namely, environment, training, and heritage as expressed diagrammatically in Figure 1. It may indeed be said that an individual is the result of the interaction of these three factors since he may be modified by changing any one of them. Although no one factor can possibly be omitted, the student of genetics places the emphasis upon heritage as the factor of greatest importance. Heritage, or

*' blood," expresses the innate equipment of the individual. It is what he actually is even before birth. It is his nature. It is what determines whether he shall be a beast or a man. Consequently in the diagram (Fig. 1), the triangle of life is represented as resting solidly upon the side marked "heritage" for its foundation.

Environment and training, although indispensable, are both factors which are subsequent and secondary. Environment is what the individual has, for example, housing, food, friends and enemies, surrounding aids which may help him and obstacles which he must overcome. It is the particular world into which he comes, the measure of opportunity given to his particular heritage.

Training, or education, on the other hand, represents what the individual does with his heritage and environment. Lacking a suitable environment a good heritage may come to naught like good seed sown upon stony ground, but it is nevertheless true that the best environment cannot make up for defective heritage or develop wheat from tares.

The absence of sufficient training or exercise even when the environment is suitable and the endowment of inheritance is ample will result in an individual who falls short of his possibilities, while no amount of education can develop a man out of the heritage of a beast. Consequently the biologist holds that, although what an individual has and does is unquestionably of great importance, particularly to the individual himself, what he is, is far more important in the long run. Improved environment and education may better the generation already born. Improved blood will better every generation to come.

What, then, is this "blood" or heritage? Exactly what is meant by heredity ?

2. A Definition of Heredity

Professor Castle, in his recent book on " Heredity in Relation to Evolution and Animal Breeding," has defined heredity as " organic resemblance based on descent." The son resembles his father because he is a " chip off the old block." It would be still nearer the truth to say that the son resembles his father because they are both chips from the same block, since the actual characters of parents are never transmitted to their offspring in the same way that real estate or personal property is passed on from one generation to another. When the son is said to have his father's hair and his mother's complexion it does not mean that paternal baldness and a vanishing maternal complexion are the inevitable consequences.

Biological inheritance is more comparable to the handing dow^n from father to son of some valuable patent right or manufacturing plant by means of which the son, in due course of time, may develop an independent fortune of his own, resembling in character and extent the parental fortune similarly derived although not identical with it.

So it comes about that "organic resemblance"

between father and son, as well as that which often appears between nephew and uncle or even more remote relatives, is due not to a direct entail of the characteristics in question, but to the fact that the characteristics are "based on descent" from a common source. In other words, an *' hereditary character" of any kind is not an entity or unit which is handed down from generation to generation, but is rather a method of reaction of the organism to the constellation of external environmental factors under which the organism lives.

To unravel the golden threads of inheritance which have bound us all together in the past, as well as to learn how to weave upon the loom of the future, not only those old patterns in plants and animals and men which have already proven worth while, but also to create new organic designs of an excellence hitherto impossible or undreamed of, is the inspiring task before the geneticist to-day.

3. The Maintenance of Life

So far as we know, every living thing on the earth to-day has arisen from some preceding form of life.

How the first spark of life began will probably always be a matter of pure speculation. Whether the beginnings of what is called life came through space from other worlds on meteoric wings, as Lord Kelvin has suggested ; whether it was spontaneously generated on the spot out of lifeless components ; or whether life itself was the original condition of matter, and the one thing that must be explained is not the origin of Hfe, but of the non-Hving, no one can say. Leaving aside the first speculation as untenable and the third as irrational, since it jars so sadly with what astronomers tell us of the probable evolution of worlds, the theory of spontaneous generation seems to be the last resort to which to turn.

In prescientific days this idea of spontaneous generation presented no great difficulties to our imaginative and credulous ancestors. John Milton, with the assurance of an eye-witness, thus described the inorganic origin of a lion : —

*' The grassy clods now calved ; now half appears The tawny lion, pawing to get free His hinder parts — then springs as broke from bonds, And rampant shakes his brindled mane."

(" Paradise Lost," Book VII, line 543.)

Ovid also in his ''Metamorphoses," not to mention a more familiar instance, easily succeeded in creating mankind from the humble stones tossed by the juggling hands of Deucalion and Pyrrha.

Although under former conditions on the earth it might have been possible for life to have originated spontaneously, and although it may yet be possible to produce life from inorganic materials in the laboratory or elsewhere, the exhaustive work of Pasteur, Tyndall and others effectually demonstrated a generation ago that to-day living matter always arises from preceding living matter and this conclusion is generally accepted as an axiom in genetics.

There are various methods of producing more life, given a nest-egg of living substance with which to start. Any organism, whether plant or animal, is continually transforming inorganic and dead material into living tissue. Through the process of repair, for example, an injury to a form as highly developed even as man is frequently made good, if it is not too extensive, as in the case of a skin wound.

When the intake of non-living material is in excess of the outgo, growth results, w4th the consequence that more living substance is built up than existed before. Thus a fragment of a living sponge or a piece of a begonia leaf are each sufficient to restore a duplicate of the original organism.

A process similar to the repair of the begonia leaf is that employed so effectively in the great groups of the one-celled animals and plants, the Protozoa and Protophyta, by means of which their numbers are maintained. These one-celled organisms multiply by fission, that is, by equal division into halves, and each half then grows to the size of the parent organism from which it sprang. When two daughter protozoans are thus formed, they are essentially orphans because they have no parents, alive or dead. The parental substance in such a process, along with the regulating power necessary to reorganization, goes over bodily into the next generation in the formation of the daughter-cells, leaving usually no remains whatever behind. In primitive forms of this description, continuous life is the natural order, and death, when it does occur, is, as Weismann has pointed out, accidental and quite outside the plan of nature.

In these cases it is easy to see the reason for "organic resemblance" between successive generations. Parent and offspring are successive manifestations of the same thing, just as the begonia plant, restored from a fragment of a begonia leaf, is simply an extension of the original plant.

Many modifications of the process of multiplication by fission occur, all of them, however, agreeing in the fundamental principle that the progeny resemble the parents because they are pieces of the parents.

Thus the greening apple maintains its individuality although coming from thousands of different trees, because all of these trees through the asexual process of grafting are continuations of the one original Rhode Island greening tree grown by Dr. Solomon Drowne in the town of Foster, nearly a century ago.

Again, certain fresh-water sponges and bryozoans, quite unlike any of their marine relatives, keep a foothold from year to year within their particular shallow fresh-water habitats by isolating well protected fragments of themselves in the form of gemmules and statohlasts. These structures may drop to the muddy bottom and live in a dormant condition throughout the icy winter when it would not be possible for the entire organism to survive near the surface.

In order to meet the conditions imposed by winter, however, these fragments have become so modified as temporarily to lose their likeness to the parent generation, although readily regaining that likeness when springtime brings the opportunity. The unity of two succeeding generations, although interrupted by the temporary interposition of something apparently different in the form of gemmules or statoblasts, is thus essentially maintained. The bryozoan colonies of two successive seasons in a fresh-water pond may be regarded as parts of the same identical colony, since they present an "organic resemblance based on descent," although the sole representatives of the parent colony during midwinter may be the sparks of life locked up within the statoblasts buried in the mud.

Similarly, the asexual spores of many plants, such as molds, mosses and ferns, may be regarded as gemmules reduced to the lowest terms, namely, to single cells. As in the preceding cases so in this instance the resemblance of the offspring which may arise from these spores, to the parents which produced them, is due to the essential material identity of two generations.

These illustrations of heredity in its simplest manifestations give the key to "organic resemblance" higher up in the scale. Sexual reproduction is no less plainly the direct continuation of life though in this instance tivo sporelike fragments out of one generation contribute to form the new individual of the next generation instead of one fragment. In all cases there is a material continuity between succeeding generations. Offspring become thus an extension of a single parent or of two parents, while heredity is simply "organic resemblance based on descent."

4. Somatoplasm and Germplasm

In forms that reproduce sexually there theoretically occurs a differentiation of the body substance into what Weismann terms somatoplasm and germplasm.

The somatoplasm includes the body tissues, that is, the bulk of the individual, which is fated in the course of events to complete a life-cycle and die. The germplasm, on the contrary, is the immortal fragment freighted with the power to duplicate the whole organism and which, barring accident, is destined to live on and give rise to new individuals.

The germplasm thus carries potencies for developing both germplasm and somatoplasm, while the somatoplasm, according to this conception, has only the power to reproduce more of its own kind. Moreover, the germplasm is not formed afresh in each generation, neither does it arise anew when the individual reaches sexual maturity, but it is a continuous substance present from the beginning. Although this theory of the continuity of the germplasm has been actually demonstrated in comparatively few instances, all the facts we know concerning the behavior of the germinal substance are consistent with it.

In many of the Protozoa the entire organism is possibly comparable to germplasm, but in all forms of life that are compounded of several cells the germplasm is probably set aside early in the development

INTRODUCTION of the individual, and this remains undifferentiated, or in reserve, like a savings-bank account put by for a rainy day, while the somatoplasm is expended in the immediate demands of the tissues that make up the individual.

In one instance at least, that of the nematode worm Ascaris, as confirmed by Boveri, this splitting off or isolation of the germplasm occurs with the very first cleavage of the fertilized egg into the two-celled stage, when one of the two cells forms the future germplasm, while the other differentiates by successive divisions into the animal itself.

Thus there results a continuous stream of germplasm, receiving contributions from other germ-

Germplasm V Somatoplasm

Fig. 2. — Scheme to illustrate the continuity of the germplasm. Each triangle represents an individual made up of germplasm (dotted) and somatoplasm (undotted) . The beginning of the life cycle of each individual is represented at the apex of the triangle where germplasm and somatoplasm are both present. As the indi- \ddual develops each of these component parts increases. In sexual reproduction the germplasms of two individuals unite into a common stream to which the somatoplasm makes no contribution. The continuity of the germplasm is shown by the heavy broken line into which run collateral contributions from successive sexual reproductions.

plasmal streams at the time of sexual reproduction, as shown diagrammatically in Figure 2, in which individuals are represented by triangles. From this continuous stream of germplasm there split off at successive intervals complexes of somatoplasm, or *' individuals," which go so far on the road of specialization into tissues that the power to be " born again " is lost, and so after a time they die, while the germplasm, held in reserve, lives on.

This is what is meant by saying that a father and son owe their mutual resemblance to the fact that they are chips off the same block rather than by saying that the son is a chip off the paternal block. Both somatoplasms are developments at different intervals from the same continuous stream of germplasm instead of one somatoplasm being derived from a preceding one. As a matter of fact the germplasm from which the son arises is modified by the addition of a maternal contribution, so that father and son in reality hold the same relation to each other that halfbrothers do.

From the point of view of genetics, then, the real mission of the somatoplasm, which is so marvelously differentiated into all the various forms that we call animals and plants, is simply to serve as a temporary domicile for the immortal germplasm. Thus the parent becomes as it were the "trustee of the germplasm," but not the producer of the offspring.

In the light of these preliminary explanations it is plain that the hopeful point of attack in the science of genetics must inevitably be the germplasm which is the source, or point of departure, in the formation of each new individual, rather than the somatoplasm, which represents the end stages of the hereditary processes.

This has not been the method of the past. The resemblances of the visible father and son have usually been traced instead of the character of their unseen germplasms. By following this old method, investigators have often been misled because the visible or apparent is not always the true index of what lies behind it. A gray and a white rabbit, for example, may produce some offspring that are entirely black just as two white-flowering sweet peas when crossed may sometimes produce purple blossoms. Consequently it is a great fallacy to affirm that in heredity *'like produces like," since the opposite is quite often the case.

The new heredity, embodied in the science of genetics, attempts to go deeper than the surface appearance of the somatoplasm. It aims to get at the source or origin of organisms, that is, the germplasm which is the only connecting thread between succeeding generations of living forms. It is concerned not so much with somatoplasm, which represents what the germplasm has done in the past, as with the germplasm and what it can do in the future.

CHAPTER n

THE CARRIERS OF THE HERITAGE 1. Introduction

Heredity, as has been shown in the introductory chapter, is essentially a matter of continuity between succeeding generations of living organisms. This continuity may be direct, as when a mother protozoan divides into two daughters, or it may be indirect, as illustrated by the relationship of a father and son, an uncle and nephew, or any other relatives of varying degrees of kinship which, taken singly or collectively, are somatoplasms derived from a common stream of germplasm.

It is the purpose of the present chapter to consider this material continuity between succeeding generations and to discover, if possible, just what are the carriers of the heritage from one generation to another. To this end it will be necessary in the first place to take up what is meant by the "cell theory."

2. The Cell Theory

In 1838-1839 the "cell theory" of Schleiden and Schwann, which aflBrms that all organisms, both plant and animal, are made up of cellular units, had its birth.

Robert Hooke, as early as 1665, had described *' little boxes or cells distinguished from one another" which he saw in thin slices of cork, and to him is due the rather unfortunate use of the term "cell" which has survived in biological writings to this day. The reason this term is unfortunate is because walls, which are ordinarily the characteristic feature of any cell, such as a prison cell, are usually the least important part of the structure of a living cell, often indeed being entirely absent.

3. A Typical Cell

A typical undifferentiated cell is represented diagrammatically in Figure 3. Near the center of the cell the nucleus is shown surrounded by a

Cefl wall

C«^to plasm Centrosome Nuclear membrane

--Nucleus .-Chromatin network

Fig. 3. — Diagram of a typical cell.

nuclear membrane. The nucleus, in common with the enveloping cytoplasm, is made up of living substance called protoplasm (Hugo von Mohl, 1846), and around the whole there is usually formed a wall or membrane which serves to separate one cell from another. Within the protoplasm there may be a considerable amount of non-living substance in the form of salts, pigments, oil-drops, water, and other inclusions of various kinds.

The nucleus is to be regarded as the headquarters of the whole cell, since changes which the cell undergoes seem to be initiated in it, while cells deprived of their nuclei cannot long survive. A single instance will serve to show the vital part which the nucleus plays in the life-history of the cell. In 1883, Gruber found that after rocking a thin cover-glass back and forth in a drop of water containing a collection of the protozoan Stentor, which has a long chain-like nucleus, these tiny animals could thus be cut into fragments, which would in some instances recover from the operation and regenerate into complete individuals. Only those pieces, however, which contained a fragment of the nucleus regenerated into new Stentors, while pieces of relatively large size which lacked a fragment of nuclear substance verv soon disintegrated.

The nucleus, it should be said, is made up of more than one substance, a fact that is easily demonstrated by processes of staining, in which certain dyes, through chemical union, stain a part but not the whole of the nuclear substance. The part most easily stained is called chromatin, that is "colored material," and during certain phases of cell life the chromatin masses together within the nucleus into visibly definite structures or bodies termed chromosomes.