282 



GENETICS 



blindness are represented. Underscored X represents 

 a color-blind determiner linked to a sex chromosome. 



From this diagram, which agrees substantially with 

 the facts, it is apparent that a color-blind male mated 

 to a normal female will produce no color-blind off- 

 spring, although the females will be "carriers" of 

 color-blindness, that is, will possess the factor in sim- 

 plex form and will, therefore, carry it for the female 

 in a latent condition. 



The sons of such a mating having a normal mother 

 and a color-blind father will be absolutely free from 

 the defect and cannot produce color-blindness in any 

 of their offspring when mated with a normal strain. 

 If, however, the "carrier" daughters from such a 

 parentage, who are genotypically heterozygous for 

 color-blindness but phenotypically normal, mate with 

 normal individuals, the expectation is that one half 

 of the sons, and none of the daughters will be color- 

 blind, but that one half of these daughters will carry 



