CHAPTER IV 

 SEX INHERITANCE 



There are two types of sex inheritance known in 

 those species in which separated sexes exist. In one 

 type, which may be called the Drosophila type (XX- 

 XY type, or, for short, the XY-type), the female is 

 homozygous for a sex factor, the male heterozygous; 

 in the other, the Abraxas type (the WZ-ZZ type, or, 

 for short, the WZ type) the female is heterozygous for 

 a sex factor, the male homozygous. Since in both 

 cases the heterozygous individuals must always mate 

 with the homozygous ones there should result in each 

 succeeding generation equal numbers of heterozygous 

 and homozygous individuals, and so the bisexual con- 

 dition is perpetuated as follows: 



so 



The genetic evidence so far gained has placed in the 

 Drosophila type the following animal forms: Dro- 



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