114 MENDELISM chap. 



condition is due to the presence of an extra factor 

 as compared with the normal, and that a single dose 

 of it will produce colour-blindness in the male but 

 not in the female, will cover a good many of the 

 observed facts (cf. Fig. 26). Moreover, it serves to 

 explain the remarkable fact that «//the sons of colour- 

 blind women are also colour-blind. For a woman 

 cannot be colour-blind unless she is 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 fmx 

 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 



