CHAPTER XI 



SEX (continued) 



THE cases which we have considered in the last chap- 

 ter belong to a group in which the peculiarities of in- 

 heritance are most easily explained by supposing that the 

 female is heterozygous for some factor that is not found in 

 the male. Femaleness is an additional character super- 

 posed upon a basis of maleness, and as we imagine that 

 there is a separate factor for each the full constitutional 

 formula for a female is FfMM, and for a male ffMM. 

 Both sexes are homozygous for the male element, and the 

 difference between them is due to the presence or absence 

 of the female element F. 



There are, however, other cases for which the explana- 

 tion will not suffice, but can be best interpreted on the 

 view that the male is heterozygous for a factor which is 

 not found in the female. Such a case is that recently 

 described by Morgan in America for the pomace fly 

 (Drosophila ampelophila). Normally this little insect 

 has a red eye, but white eyed individuals are known to 

 occur as rare sports. Red eye is dominant to white. 

 In their relation to sex the eye colours of the pomace fly 



