An Introduction to Modern Genetics

C. H. Waddington

• Publisher: Routledge
• Year: 2016
• Topic: History
• eBook ISBN:
  9781317352037
• Subtopic: World History
• Index: History

First published in 1939 (second impression in 1950), this book provides an account of the changes in, and main principles of, genetics at that time. These are illustrated by references to the most authoritative and then recent investigations. Special attention is paid to the way in which genetics overlaps with other fields of inquiry, since it is often in these border-line subjects that the most important advances are to be expected. The book is particularly arranged to suit the convenience of students whose previous knowledge of genetics is small, and contains annotated bibliographies of suggestions for further reading.

Part One
Formal Genetics

The first task of genetics was to discover the rules of inheritance. The young science, beginning its life at the time Mendel's papers were rediscovered [1900], accomplished this task extremely quickly; within twenty years, the chromosome theory had been placed on a firm basis and the main outlines of the science of heredity were clear. This development is dealt with in Part I. The first chapter is devoted to the basic facts on which the chromosome theory rests, the next two to the consequences of chromosome heredity at different stages of the chromosome cycle and in abnormal chromosome cycles. The fourth chapter describes peculiarities in the behaviour of whole chromosomes, while the fifth deals with the parts of chromosomes. The study of the mechanism of crossing-over, considered in Chapter 6, is probably the most rapidly advancing part of formal genetics at the present time; it is one of the most important lines of attack in the investigation of the nature of the cell and its parts, and it links up with the study of the physicochemical nature of the gene to which we return in Part V.

Chapter 1
The Fundamentals of Mendelism

1. Biological inheritance; genotype and phenotype

It has of course been dear from very early times that there is some biological inheritance. The character of an organism depends, to some extent at least, on that of its parents. The problem of how the parental influence is exerted was for long debated in purely philosophical terms, but with the invention of adequate microscopes, and the discovery of spermatozoa and of the universal occurrence of eggs, hypotheses of a verifiable nature could be put forward. The first theory to gain general acceptance had a deceptive air of simplicity; it was supposed that the sperm (or, for the feminists, the egg) contained the complete organism in miniature, which merely had to grow to become the new adult. Elaborate theories were evolved as to how these homunculi in their turn mated and reproduced; but still more efficient microscopes soon made it clear that the eggs and sperm do not in fact contain miniature animals, and the whole elegant edifice of theory had to be abandoned.¹

Since we can easily find examples of an animal inheriting, say, the colour of its eyes from its father; and since there are no eyes in the sperm, which is the only connection between the two individuals, it is clear that the eye colour must be represented in the sperm by something else which is responsible for passing on the father's characteristic to his son. We must therefore draw a distinction between the characters of an adult individual and the representatives of those characters which are present in the germ-cells and pass on into the next generation. The former are known collectively as the phenotype, the latter as the genotype; but these two terms will need some further discussion, since they were invented after the basic theory of genetics had been developed, and are to some extent coloured by its conceptions.

The fundamental step in the understanding of heredity depended on a bold piece of abstract thinking.² To the unanalytical eye of common sense, biological inheritance is singularly capricious; in some cases an animal may be more like its father, in other cases more like its mother, while in some respects it may not be like either of them. In general, one cannot easily pretend that organisms are simply intermediate between their two parents. It seems that one must conclude that an organism does not inherit the whole of its parents' genotype, but only part of it; in any given case some of the characters which might have been inherited from the parent actually fail to appear. But theory went no farther than this till Mendel was bold enough to leave out of consideration the greater part of the characteristics of the organisms with which he was working and to concentrate entirely on one or two sharply marked features. In this way he proceeded to consider the phenotype (the appearance of the adult animal) as a set of elementary characters; and although it is obvious that an organism is not a mere assemblage of isolated anatomical structures, this analysis revealed the fundamental fact that the genotype consists of hereditary units which are very largely independent. The relation between the genotype and the phenotype, that is to say, between the hereditary constitution and the appearance of an organism, can therefore only be understood after we have discussed the facts which have been revealed by Mendelian analysis.

2. Mendel’s First Law: Factors and their Segregation

Mendel's fundamental discoveries were made on the garden pea, Pisum sativum. The typical, oft-quoted experiment was as follows: tall-growing pea plants were crossed with short plants (P1, the first parental generation); their offspring (F1, first filial generation) were all tall, and when self-fertilized, gave a second generation of hybrids (F2) consisting of tall and short plants in the ratio of three to one. The short plants from the F2 bred true for shortness when selfed, and a third of the talls bred true for tallness, while the other two-thirds of talls again gave talls and shorts in the three to one ratio which had been found in the F2. Mendel's hypothesis was this: tallness and shortness are dependent on a pair of alternative factors, which we may call T tall and t short. Each fertilized zygote, and each cell of the organism into which it develops, contains two of these factors, and may thus be TT, or Tt or tt; but the gametes each contain only one factor selected out of the two which are contained in the germ-mother cell out of which the gamete is formed. The first cross was between TT and tt, and gave a F1 of Tt; the fact that this F1 shows as tall plants must mean that during development the T factor "dominates" over the t, which is said to be "recessive." When the F1 is selfed, each Tt plant forms equal numbers of gametes with T and with t, and if these unite at random they will give TT, Tt and tt plants in the ratio 1: 2:1. Thus there will be three talls (of which one will be pure breeding TT and two Tt like the F1) to one pure breeding tt short. (Fig. 1.)

Factors of the kind postulated above are called Mendelian factors or genes. A collection of genes alternative to one another, so that normally a gamete contains only one of the set, is spoken of as a series of allelomorphic factors (the position they occupy is their "locus"); there may be more than two members of such a series, for instance, there might have been another alternative "dwarf" for Mendel's peas. Zygotes which contain two similar allelomorphs are said to be homozygous for the gene in question (e.g. the pure breeding TT or tt), while if the two allelomorphs are dissimilar, as in the F1 plants, the organism is heterozygous. If, in a heterozygote, only one of the two allelomorphs has an effect on the character of the organism, that allelomorph is dominant over the other, which is recessive; dominance and recessiveness may be partial, when the heterozygote will show some effect of both genes, neither of which completely suppresses the other.

Fig. 1. Mendel's experiment of crossing tall and short peas.

The fundamental points of the hypothesis developed above, which is known as Mendel's first law, are (1) that there are factors which affect development and that these factors or genes retain their individuality from generation to generation and do not become contaminated when they are mixed in a hybrid, and (2) that they become sorted out from one another when the gametes are formed.

A Note on Symbols.— There are several different systems in use for symbolizing genes. The simplest refers to a locus by a certain letter, and indicates the dominant by the capital, the recessive by the lower case letter (e.g. Aa for the heterozygote). If there are more than two allelomorphs, they can be indicated by small superscript letters (e.g. c^a, c^r, c^k, etc., for the albino series in rodents). The convention of indicating dominance by capital letters breaks down here since there is more than one dominance relation to be considered, and anyway the dominance within such series is usually incomplete. Sometimes the most common allelomorph in the series and those dominant to it are given capital letters, those recessive to it small letters. Alt...

Table of contents

• Cover
• Title
• Copyright
• Original Title
• Original Copyright
• Contents
• PART ONE FORMAL GENETICS
• PART TWO GENETICS AND DEVELOPMENT
• PART THREE GENETICS AND EVOLUTION
• PART FOUR GENETICS AND HUMAN AFFAIRS
• PART FIVE THE NATURE OF THE GENE
• Appendix LABORATORY METHODS FOR CLASS-WORK ON DROSOPHILA
• BIBLIOGRAPHY AND AUTHOR-INDEX
• SUBJECT INDEX