CHAPTER I 



The Fundamentals of MendeKsm 



I. Biological inheritance; genotype and phenotype 



It has of course been clear 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 simpUcity; 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.^ 



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 some- 

 thing 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 geno- 

 type; 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.^ 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 it mother, 

 while in some respects it may not be like either of them. In general, one 



^ Cf. Punnett 193 1. For a general history of embryology, cf. Needham 19345 

 for a chronology of genetics 1 800-1934, Cook I937. 



^ For a logistic analysis of genetical theory, see Woodger 1938. 



