Color 'blindness 261 



Sex-linked Characters 



Among fruit flies there appears from time to time a 

 male fly having white eyes in contrast to the usual dark red 

 of the wild forms. If such a white-eyed male is mated with 

 the typical red-eyed female, all the offspring have red eyes. 

 If these hybrids are interbred there will be a segregation 

 among the progeny yielding three red-eyed offspring to one 

 white-eyed. Strangely enough, however, white eyes occur 

 only among the males. The explanation for this is found in 

 assuming that the dominant eye-pigment determiner is here 

 present in the X chromosome (Fig. 73). All the females 

 have red eyes because each has at least one X chromosome 

 containing the factor that results in pigment. Only half the 

 males can have red eyes because only half of them can receive 

 such an X chromosome, the others receiving the recessive X 

 chromosome. This interpretation is confirmed by making a 

 reciprocal cross (see Fig. 74). 



Color-blindness 



The association between special determiners and the sex 

 chromosomes has been established for other characters and 

 for other species, including man. Color-blindness of a cer- 

 tain type is found among human beings to be such a sex- 

 linked character (see Fig. 75). Whatever it is that causes 

 one to be color-blind seems to be carried by the X chro- 

 mosome, of which the male has^ one and the female two. 

 This does not mean that every X chromosome carries the 

 determiner for this trait, but only that when it is present it 

 is associated with the X chromosome. As in the white eye- 

 color of the Drosophila male, the sex-linked character color- 

 blindness in man is transmitted from the male through a 

 daughter (who does not show it) to a grandson. Color- 

 blind females are very rare; but such would occur in the 

 offspring of a color-blind father whose wife was the daughter 

 of a color-blind man (Fig. y6) . 



