XI SEX 117 



without M and the ova containing F. In other words we 

 must on this view suppose that fertiHsations between 

 certain forms of gametes, even if they can occur, are in- 

 capable 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 reptdsion 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 pecuHar form of 

 sex limited inheritance in man. r? y f 



It has long been a matter of com- | 



mon knowledge that colour-blind- $ ^ ? ^ (j ■ 



ness is much more common among '7~L L . L ' ' 

 men than among women, and also ^^^ ^^ 



that unaffected women can trans- scheme to aiustrate the probable 



, , - . \ jl J. niode of inheritance of colour- 



mit it to their sons. At nrst blindness. The dark signs re- 



. . 2. ^'^ present affected individuals. 



sight the case is not UmiKe a black dot in the centra de- 



^ 1_ j-U notes an unaffected female who 



that of the sheep, where tne is capable of transmitting the 



, _ _ . , . condition to her sons. 



homed character 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 



