loS 



MENDELISM 



CHAP. 



if they can occur, are incapable of giving rise to 

 zygotes with the capacity for further development. 

 If we admit this supposition, the scheme just 

 given will cover such cases as those of the currant 

 moth and the fowl, equally as well as that of the 

 pomace fly. In the former there is repulsion between 

 either the grossulariata factor and F, or else between 

 the pigment inhibitor factor and F, while in the 

 latter there is repulsion between the factor for red 

 eye and M. 



Whatever the merits or demerits of such a 

 scheme it certainly does offer an explanation of a 

 peculiar form of sex limited 

 inheritance in man. It has 

 long been a matter of common 

 knowledge that colour-blind- 

 ness is much more common 

 among men than among 

 scheme to illustrate the probable women.and also that unaffected 



mode of inheritance of colour- 



blindness. The dark signs re- 



present affected individuals. 

 ' A black dot in the centre de- 



notes an unaffected female who n _f. nnliW fViaf r>f fVif* 

 is capable of transmitting the 1S nOt UnllKC tHat . OI trie 



sheep, where the horned char- 



acter is apparently dominant in the male but recessive 

 in the female. The hypothesis that the colour-blind 

 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 all the 

 sons of colour-blind women are also colour-blind. 

 For a woman cannot be colour-blind unless she is 



n , r . mf ^ n ~ Qrl francmi'f if f/-> 



women can transmit it to 



crvrlo A f. f; rcr cirrVif tVif* 



S>OnS. At HrSt SlgHt HlC 



