x] Heredity of Colour-Blindness 173 



the facts respecting the genetics of colour-blindness. The 

 transmitting daughters of colour-blind fathers are evidently 

 heterozygotes in whom the affection is, like the horned 

 character in the F^ ewes, recessive ; but in the fact that 

 both the sons and their offspring are free from the affection 

 (unless of course it be introduced by the mother) we meet 

 a fresh complication the meaning of which is considered 

 later (p. 195, note]. Here it must suffice to say that the 

 most obvious test of the nature of the inheritance is pro- 

 vided by the families of colour-blind women. According 

 to the scheme the simplest expectation is that all sons of 

 such women should be colour-blind. Up to the present 

 time we have records of seven colour-blind women only who 

 had sons. In all they had 17 sons who lived to be tested, 

 and all were found to be colour-blind^. We may therefore 

 rest assured that the scheme provides at least a substantial 

 part of the truth, and that colour-blindness is a condition 

 produced by the addition of a dominant factor. 



In the examples just considered sex itself acts as a 

 specific interference, stopping or inhibiting the effects of a 

 dominant factor, and it is not a little remarkable that the 

 inhibition occurs always, so far as we know, in the female, 

 never in the male. When the effects of a factor fail to 

 appear in a zygote the failure is due to one of two causes. 

 Either some complementary element is absent which is 

 needed to produce the effect, or some other element is 

 present which inhibits it. The facts scarcely enable us to 

 distinguish between these two possibilities but we may feel 

 some confidence that our cases belong not to the first 

 group but to the second. For since the condition can be 

 developed in females, it is evident that maleness itself is 

 not a necessary complement ; and it is not easy to suppose 

 that there is some other factor regularly coupled with male- 

 ness which has this property, though that possibility cannot 

 be absolutely excluded. The suggestion however that the 

 female contains something which suppresses the effect of the 

 otherwise dominant factor is consistent with the observation 

 that when these sex-limited conditions, as they are called, 

 do appear in females, they are developed to a somewhat 

 less degree than in males, just as in horned breeds of sheep 

 the ewes have horns smaller than those of the rams. The 



* See p. 224. 



