IX] HEREDITY AND SEX 127 



his daughters, for sons of aflected men are normal 

 and do not transmit, while the daughters of affected 

 men, though not themselves affected, transmit the 

 disease to some of their sons. 



The suggested explanation is that ' normality ' is 

 represented by a factor for which the female is 

 homozygous (NN), the male heterozygous (Nil). If 

 in the male the normality factor N is replaced by the 

 factor for colour-blindness, for example (C), it will 

 cause the appearance of the disease in the male for 

 there is no second N to counteract C. The male 

 transmits N or C only to his daughters, but a woman 

 who receives C from her father will receive N from 

 her mother, and so will not be colour-blind. She will 

 transmit C, however, to half her children, so that 

 some of her sons will be colour-blind. This is illus- 

 trated in the following scheme. 



? NN X ^ On 



normal woman //\ colour-blind man 



X \ 



%CN X ^ Nil 



heterozygous /\ normal man 

 woman y \ 



%CN'%NN ^ Cn J^i 



In the fly Drosophila and in Man, then, it is clear 

 that the germ-cells of the male must be of two kinds, 

 while we have seen that precisely similar evidence 



