HEREDITY 



of white eyes or short wings in Drosophila, 

 rather than of barring in poultry. 



Color-blindness is much commoner in men 

 than in women. A color-blind man, however, 

 does not transmit color-blindness to his sons, 

 but only to his daughters, the daughters, how- 

 ever, are themselves normal provided the 

 mother was; yet they transmit color-blindness 

 to half their sons. A color-blind daiUghter 

 could be produced, apparently, only by the 

 marriage of a color-blind man with a woman 

 who transmitted color-blindness, since the 

 daughter to be color-blind must have received 

 the character from both parents, whereas the 

 color-blind son receives the character only 

 from his mother. 



Color-blindness is apparently due to a defect 

 in the germ-cell, — absence of something nor- 

 mally associated there with an X-structure, 

 which is represented twice in woman, once in 

 man. Color-blindness follows, therefore, in 

 transmission the scheme shown in Fig 53. 



If, as has been suggested, the determination 

 of sex in general depends upon the inheritance 

 of a Mendelian factor differentiating the sexes, 



180 



