CHAPTER X 



SEX {continued) 



The facts with which we dealt in the last chapter 

 are all in accordance with the hypothesis that the 

 female is heterozygous for a given sex factor which 

 is lacking in the male. The peculiar phenomenon of 

 sex -limited heredity enables us to decide that in 

 birds and moths the male is the homozygous and 

 the female the heterozygous sex. It is a very 

 curious fact that in certain other groups of animals 

 this position is apparently reversed, the male being 

 the heterozygous and the female the homozygous 

 sex. This was first discovered by Morgan in one 

 of the earliest of his remarkable series of experiments 

 with the little pomace fly {Drosophila ampelophila\ 

 about which we shall have more to say later on. 

 The wild Drosophila has a red eye. In certain of 

 Morgan's cultures a few white-eyed males appeared. 

 These were mated with normal red-eyed females, 

 and gave only red - eyed offspring. In the F^ 

 generation reds and whites were produced approxi- 

 mately in the ratio 3:1, but all of the whites were 

 males. Further breeding tests showed that all of 

 the red-eyed males were heterozygous, while of the 



