GENETIC THEORY 5 



establishment of the characteristic chromosome number. 

 One member of each of the pairs of chromosomes with 

 which the new individual is endowed comes by way of the 

 spermatozoon from the father; its mate comes by way of 

 the ovum from the mother. 



Many of the details of structure and function that are 

 exhibited by the individual are the expression of the heredi- 

 tary constitution of the individual. Mendel postulated that 

 such characters were brought into being by the action of 

 'hereditary factors'. In every individual's constitution, and 

 in respect of any given character, there were two such factors, 

 only one of which passed into each gamete. Commonly, of 

 the pair of hereditary factors one was dominant, the other 

 recessive, the dominant one alone exerting an influence dur- 

 ing development. Thus an individual exhibiting the dom- 

 inant member of a contrasted pair of hereditary characters 

 could be either a homozygous dominant (DD) or else a 

 heterozygous dominant (Dd). 



Mendel (1865) himself hazarded the suggestion that sex- 

 determination might prove to be a phenomenon of heredity 

 and segregation. Experimental evidence of its validity was 

 furnished as early as 1907 by Correns, who studied hybrids 

 between monoecious and dioecious species of Bryonia. His 

 results indicated that in the dioecious species sex was deter- 

 mined by the pollen grain of which there were two kinds 

 equal in number, one being male-producing and the other 

 female-producing whilst the ovules were all of a kind. 

 Correns compared the combinations resulting from the 

 union of the two forms of pollen grain with the one form of 

 ovule with those of the typical back-cross of a Mendelian 

 experiment in which the heterozygous dominant (Dd) 

 mated to a recessive (dd) yields equal numbers of hetero- 

 zygous dominants and recessives. 



Doncaster (1906), working with the currant moth Abraxas 

 grossulariata, produced evidence that strongly suggested 



