No. 575] AN APTEROUS DROSOPHILA 685 



ratio of heterozygotes to pure blacks in the winged class 

 shows that the flies which should have been gray accord- 

 ing to expectation have been added to the heterozygotes. 

 Likewise the gray flies in the apterous class are about four 

 times as numerous as anticipated, showing that the ex- 

 pected heterozygotes and blacks are here gray. From this 

 it is evident that the factors a P and B, on the one hand, 

 and A P and b, on the other, have remained associated in 

 the combination which they formed in the parents, instead 

 of independently segregating. Such an explanation ac- 

 counts for the absence of A P B and a P b gametes in the F l 

 generation, and consequently for the absence of gray, 

 winged flies, and of black or heterozygous apterous flies 

 in F 2 . The evidence accords with that obtained for many 

 other mutant characters in Drosophila, and the explana- 

 tion is the same as that given for the previous cases (e. g., 

 Morgan, 1911, 1912 ; Morgan and Lynch ; Sturtevant, 1913 

 a and b; Dexter). 



TABLE VI 



The presence of a definite linkage or association be- 

 tween apterous and black (/. e., between either Op or A v 

 and b or B depending upon the nature of the cross) as 

 shown by this experiment, together with the absence of 

 any such linkage with characters in Groups I and III, as 

 shown by the preceding experiments, indicate that apter- 



