XI SEX 113 



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 

 poniace fly. In the moth there is repulsion between 

 either the grossulariata factor, and F, in the fowl 

 between the pigment inhibitor factor and F in the 

 female, while in the fly there is repulsion between 

 the factor for red eye and M in the male.^ 



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 C5 X T 



long been a matter of common q ^ 5 "X ^ , 

 knowledge that colour-blind- | ' *^ 



ness IS much more common J af r? § 2 

 among men than among „ 



women, and also that unaffected scheme to illustrate the probable 

 women can transmit it to their Sind\l\"'?i:eXk1i^°n'sT 

 sons. At first sight the case, TbTal doHf thi"Se"de: 

 is not unlike that of the rs^rofrnShlrthe 

 sheep, where the horned char- condition to her sons. 

 acter is apparently dominant in the male but recessive 

 in the female. The hypothesis that the colour-blind 



1 Still more recently Doncaster {^Journal of Genetics, 191 1) has 

 proposed that the female should be regarded as FfMM and the male 

 2i%jfMm, each sex being heterozygous for its own sex factor, while 

 the female is also homozygous for that of the male. The view has 

 much to commend it, for it offers a basis of explanation for the not 

 infrequent assumption of male characters by females which are old or 

 suffering from ovarial disease. In such cases it may be supposed that 

 the factor F in some way loses its potency and allows the underlying 

 M factors to come into fuller play. In its general working the hypo- 

 thesis resembles that given in the text. 



I 



