x] Heredity of Colour-Blindness 173 



respecting the genetics of colour-blindness. The course of 

 the descent of such a sex-limited condition is represented 

 diagrammatically in Fig. 33. 



Many cprollaries follow from this account, and by the 

 way in which the facts obey them the truth of the propo- 

 sition may be tested. In connection with the special subject 

 of Mendelian heredity in man further details will be given. 

 Here it must suffice to say that the most obvious test of 

 the nature of the inheritance is provided 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 

 1 7 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 close approximation to 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 



