182 MULTIPLE FACTORS 



Such flies were actually obtained, although they were 

 rather dark in color, since both black and ebony 

 produce some effects on flies heterozygous for them. 

 In F2 the expectation is 9 gray, 3 black, 3 ebony, 

 and 1 black ebony (double recessive). When F2 

 was actually obtained it was found to be impossible 

 to make an accurate separation of the four classes. 

 There was a practically complete series ranging from 

 the normal gray to individuals darker than either 

 black or ebony. The gradation is obviously due 

 chiefly to the fact that dominance is not complete. 

 There are nine different classes expected, instead of 

 four, if heterozygous forms be counted. These nine 

 classes form groups, each with its own mode, the 

 outlying members of each group overlapping neigh- 

 boring groups. To add to the difficulty, the colors 

 change considerably with the age of the fly. There 

 are at least seven other mutant factors known in 

 Drosophila that make the flies darker. It will 

 readily be seen that, if one had a population contain- 

 ing a mixture of all these characters, analysis would be 

 well-nigh impossible. 



Before making the above cross the inheritance of 

 black and of ebony had been studied separately, and 

 no difficulty in classification is encountered unless 

 they are used in the same cross. This information 

 made it possible for us to interpret the black ebony 

 cross. In the experiments now to be described, we 

 are dealing with factors which had not first been 

 studied separately, so that the interpretation is not so 

 obvious as in the preceding case. 



