76 Heredity and Eugenics 



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 apparently come 

 about as follows: 



FIRST CASE 



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



Gametes of white-eyed male = X and 



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



SECOND CASE 



Gametes of white-eyed female =X and X 



Gametes of red-eyed male =X-R and 



Zygotes = X X-R (red-eyed female), and >X (white-eyed male). 



A short condition -of the wings in Drosophila, which 

 renders the animal incapable of flight, is likewise sex 

 limited in heredity, as has been shown by Morgan. By 

 crossing two races of Drosophila, each of which possessed a 

 different sex-limited character, Morgan has been able to 

 combine the two characters in a single race. Thus was 

 obtained a race both white eyed and short winged. The 

 synthesis cannot be made originally in a male individual, 

 but only in a female. For only in the female can the two 

 characters be brought together, each associated with a 

 different X, since in the male only one X is present. Al- 

 though each sex-limited character seems to be attached 

 to or bound up with an X structure, it evidently has a 

 material basis distinct from X. Otherwise it would not 



