HEREDITY 



words, the female may be homozygous as re- 

 gards a sex-limited character, but the male can 

 only be heterozygous (see Fig. 53). 



Drosophila normally has red eyes, but the 

 redness of the eye is a distinct unit-character, 

 sex-limited in heredity. Further males are 

 regularly heterozygous in this character, while 

 females are homozygous. For Morgan has ob- 

 tained a race in which the eyes are white, 

 owing to the loss of the red character; and 

 reciprocal crosses of this race with ordinary 

 red-eyed animals yield different results. The 

 red-eyed female crossed with a white-eyed male 

 produces only red-eyed offspring, but the red- 

 eyed male crossed with a white-eyed female 

 produces offspring only half of which are red- 

 eyed, viz. the females, whereas the males are 

 white-eyed. 



These different results in the two cases ap- 

 parently come about as follows: 



First case. 



Gametes of red-eyed female = X-R and X-R, 

 Gametes of white-eyed male = X and , 

 Zygotes= XX-R (red-eyed female), and = -X-R 

 (red-eyed male). 



176 



