HEREDITY OF BODY COLOR IN DROSOPHILA 33 



could occur may not appear clear at first sight, for both the 

 wild gray females and the gray-black hybrids contain two doses 

 of black — the only difference between the two is the absence 

 of one dose of yellow, in the hybrids. If, in the absence ' of 

 this dose of yellow, the black has a better chance to show itself 

 more positively, we can account for the intermediate char- 

 acter of the hybrid. If such is the case, the yellow factor is to 

 some extent a partial inhibitor of black. The same explanation 

 applied to the males is as follows: the wild gray male has only 

 one dose of black (since black is sex-linked). It has two doses 

 of yellow. The hybrid has also one dose of black (B) but only 

 one dose of yellow. It differs, therefore, from the wild male 

 in having one dose of yellow instead of two. The darker color 

 of the hybrid male would, in consequence, be due to relatively less 

 yellow than that present in the wild male. The explanation is 

 the same therefore for both sexes, but it involves the assumption 

 that the color of the wild female which is the same as that of 

 the male is due to the presence of a double dose of B and Y(BY, 

 BY) while the color of the wild male is due to one dose of black 

 and two of yellow (BY, Y). Removal of one Y from the wild 

 female makes her darker; and similarly the removal of one Y 

 from the wild male makes him darker also. 



The numerical results for the Fo generation are as follows: 



[Gray 9 5053 



n ^ u n ^ /G 9 J Gray c? 48G1 



G9byBd^= |^^= jj^j^^j^ ^ ^280 



[Black d" 1385 



The expectation, as shown by the analysis below, calls for 

 three grays to one black. There are 9914 grays and 2665 blacks. 

 The blacks fall considerably below expectation, yet the black 

 flies are a vigorous strain and appear in the cultures to breed 

 as well as the grays. 



It will be noted that while the gray males run some 200 flies 

 behind the females, the black males exceed the females by 100 

 flies. 



Since these counts are from more complex crosses involving 

 white eyes and short wings as well as red eyes and long wings, 



