xi SEX 109 



homozygous for the colour-blind factor, in which case 

 all her children must get a single dose of it even if 

 she marries a normal male. And this is sufficient 

 to produce colour-blindness in the male though not 

 in the female. 



But there is one notable difference in this case as 

 compared with that of the sheep. When crossed 

 with pure hornless ewes the heterozygous horned 

 ram transmits the horned character to half his 

 male offspring (cf. p. 71). But the heterozygous 

 colour-blind man does not behave altogether like a 

 sheep, for he apparently does not transmit the colour- 

 blind condition to any of his male offspring. If, 

 however, we suppose that the colour-blind factor is 

 repelled by the factor for maleness, the amended 

 scheme will cover the observed facts. For, denoting 

 the colour-blind factor by X, the gametes produced 

 by the colour-blind male are of two sorts only, viz. 

 Mfx and mfX. If he marries a normal woman 

 (Ffmmxx), the spermatozoa Mfx unite with ova Jmx 

 to give normal males, while the spermatozoa mfX 

 unite with ova Fmx to give females which are 

 heterozygous for the colour - blind factor. These 

 daughters are themselves normal, but transmit the 

 condition to about half their sons. 



The attempt to discover a simple explanation of 

 the nature of sex has led us to assume that certain 

 combinations between gametes are incapable of 

 giving rise to zygotes which can develop further. 

 In the various cases hitherto considered there is no 

 reason to suppose that anything of the sort occurs, 

 or that the different gametes are otherwise than 

 completely fertile one with another. One peculiar 



