CHAPTER X 



SEX {continued^ 



The facts with which we dealt in the last chapter 

 are all in accordance with the In'pothesis that the 

 female is heteroz\'gous 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 homozy^gous 

 sex. This was first discovered by IMorgan in one 

 of the earliest of his remarkable series of experiments 

 with the little pomace fly {Drosop/tiia anipelopJiihi), 

 about which we shall ha\e more to say later on. 

 The wild DrosopJiila 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 w^ere produced approxi- 

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

 males. Further breeding tests showed that all of 

 the red-eyed males were heterozygous, while of the 



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