106 EVOLUTION AND GENETICS 



consist of white-eyed and red-eyed females and 

 white-eyed and red-eyed males. The ratio here is 

 1 : 1 and not three to one (3:1) as in other Mendelian 

 cases. But Mendel's law of segregation is not trans- 

 gressed, as the preceding analysis has shown; for, 

 the chromosomes have followed strictly the course 

 laid down on Mendel's principle for the distribution 

 of factors. The peculiar result in this case is due to 

 the fact that the F^ male gets his single factor for 

 eye color from his mother only, and it is contained in 

 a body (the X-chromosome) that is involved in sex- 

 determination, while the mate of this body, the Y- 

 chromosome, is indifferent with regard to these fac- 

 tors. 



In human inheritance there are characters that 

 show this same kind of transmission. Color-blindness, 

 or at least certain kinds of color-blindness, appears 

 to follow the same scheme. A color-blind father 

 transmits through his daughters his peculiarity to 

 half of his grandsons, but to none of his grand- 

 daughters {fig. 69). The result is the same as in the 

 case of the white-eyed male of Drosophila. Color- 

 blind women are rather unusual, which is expected 

 from the method of inheritance of this character, 

 but in the few known cases where such color-blind 

 women have married normal husbands all thel^i sons 

 inherit color-blindness from the mother (fig. 70). 

 Here again the result is the same as for the similar 

 combination in Drosophila. 



