280 



GENERAL ZOOLOGY 



sons. Figure 168 shows the results of the mating of a color- 

 blind mother and a normal father. 



Crossing Over. — Many other sex-linked characters have been 

 discovered and their heredity has been intensively studied in 

 Drosophila. One should expect that if two sex-linked characters 

 are present in the same animal, these two characters would 

 remain associated in a single individual in subsequent generations, 

 since according to the theory, the genes for both characters are 



Eyes 



Chromosomes 



X X Parents 



6 9 



X 6 , 



l XI ^ I Gametes 



XX? 



<o> 



<u> 



6 



Fig. 168. — Diagram of the inheritance of color blindness through the female. 

 A color-blind female transmits the defect to all her sons, to half of her grand" 

 daughters and to half of her grandsons. Corresponding distribution of sex 

 chromosomes on the right. (From Conklin, Heredity and Environment, Princeton 

 University Press, after Morgan. By permission.) 



located in the same chromosome. This, however, is only par- 

 tially true, as may be illustrated by the following experiment. 

 Yellow wing is a sex-linked character which, when crossed with 

 its dominant allelomorph, gray wing, exhibits the same heredi- 

 tary relationship as when white eye color is crossed with red eye. 

 Its gene like that of white eye must therefore be located on the 

 X chromosome, and the same must be true for the gene for gray 

 wing. If a female having both white eyes and yellow wings is 

 crossed with a male having red eyes and gray wings, the males of 

 the F\ have yellow wings and white eyes, and the females have 

 gray wings and red eyes. Breeding the Fi flies produces in the 

 Fi four classes as follows: 



