Mechanism of Mendelian Heredity 237 



nistic units (chromosomes, characters) is contained by 

 exactly half the gametes produced. 



It is obvious that Sutton by this idea did for heredity 

 in general what McClung had done for sex determi- 

 nation or sex heredity, that is, he showed that the 

 numerical results obtained in Mendelian heredity can 

 be accounted for on the basis that factors for hereditary 

 characters are carried by definite chromosomes. The 

 cytological basis of sex determination becomes only a 

 special case of the cytological basis of Mendelian 

 heredity. In the examples quoted the plants giving 

 rise to violet and to white flowers are homozygous 

 for the colour of flower having the chromosome constitu- 

 tion A A and aa respectively; while the plants with pale 

 violet flowers are heterozygous, having the chromosome 

 constitution Aa in their nuclei. The former give rise 

 to identical sex cells A and A or a and a; while the 

 heterozygous plants give rise to different sex cells A 

 and a. 



From this point of view in Drosophila (and very 

 probably also in man) the female is homozygous for 

 sex having in all its cells the critical chromosome 

 constitution XX and giving rise to one type of eggs 

 only, each with one X chromosome; while the male in 

 these forms is heterozygous for sex having in all its 

 cells the chromosome constitution XY and forming 

 two different types of spermatozoa in equal numbers 



