n8 MENDELISM CHAP. 



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 all 

 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 com- 

 pared 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 trans- 

 mit 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 m/X. 

 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 colourblind factor. 

 These daughters are themselves normal, but transmit the 

 condition to about half their sons. 



