HEREDITY OF BODY COLOR IN DROSOPHILA 31 



all the female offspring should be black and all the males brown. 

 In general I have found my separation to be correct. The 

 difficulty has arisen apparently in most cases with heterozygous 

 yellow females that contain only one dose of yellow instead of 

 two, as does the ordinary pure yellow. In such cases two classes 

 of females appear when crossed to black males, namely, gray 

 and black. 



In regard to the distribution in the wild fly of the products 

 of the three color factors, that go to produce its color, it is diffi- 

 cult to speak with certainty, but from comparison with corre- 

 sponding regions in the mutants when one or another of the color 

 factors is absent it appears that the black regions are due to 

 the black factor, but the brown may be present and overlaid 

 by the black. At least it may be said that the black regions 

 in the gray fly and in the black fly are brown in the brown fly, 

 but of course it is possible that when the black develops the brown 

 may not develop, or the black may even be a further stage of 

 development of the brown pigment. The yellow of the wild 

 fly also seems to replace the brown of the brown fly at least 

 when yellow is absent the color of the yellow regions is brown. 

 Possibly, as I have suggested, yellow when present inhibits 

 brown, for otherwise it is difficult to see how the yellow fly 

 should be lighter in color than the brown fly. 



The black flies are large and vigorous. There is no difficulty 

 in breeding them or in crossing. The yellows are generally 

 smaller (though not always) and are more delicate. They get 

 stuck very easily to the moist sides of the culture bottles, and, 

 being unable to free themselves, perish. They are more difficult 

 to breed and to cross. The brown flies although generally 

 large are weak. They get stuck to the food and to the walls 

 of the bottles and die. Otherwise they seem healthy. On the 

 whole the mutants are weaker than the normal flies, but the 

 loss of the yellow factor that produces the black flies is less 

 injurious then the loss of the black factor that produces the 

 yellow fly. I have at times thought that the loss of both of 

 the factors produces the weakest fly in the series — the brown 

 fly — but this is difficult to prove. 



