32 T. H. MORGAN 



Formulas 



The color of the wild fly appears to be due to the presence 

 of three factors, Black (B), Yellow (Y). and Brown (Br). For 

 brevity this color is spoken of as Gray, which corresponds nearly 

 to the color of the semi-transparent wings. If the black factor 

 (B) is absent (b) the color of the fly is yellow (Y), more especi- 

 ally the wings. The yellow fly is therefore bYBr. Where 

 the factor for yellow (Y) is absent (y) the fly is black, more 

 especially the wings. The black fly is therefore ByBr. When 

 both black and yellow are absent the fly is brown, more espe- 

 cially the wings. The brown fly is, therefore, byBr. The brown 

 fly can always be produced by crossing yellow and black and 

 inbreeding the Fi's which give by recombination some Browns 

 in the second, F2, generation. Of course, the same result would 

 follow if both yellow and black were lost from the gray fly 

 at the same time, but this is unlikely since the black and the 

 yellow factors lie in different parts of the hereditary complex. 



Of these three color factors that of black is sex-linked; the 

 yellow factor is not sex-linked, and is contained in all gametes 

 both in the male and in the female of gray and yellow flies. 

 One must be careful to observe that while the factor for black 

 is sex-linked the black fly, bred to gray does not show sex- 

 linked (sex-limited) inheritance; while the yellow fly bred to 

 normal shows sex-linked inheritance. This will be clear from 

 an examination of the analyses given below. 



All possible crosses have been made between these mutants, 

 and, these may now be taken up in order. 



Wild {gray) by black 



When the female wild flies (Gray) are mated to male black 

 flies all of the offspring are gray. These Fi gray flies are darker 

 than the wild flies, i. e., they are to some extent intermediate in 

 color between gray and black. It is true that there is much 

 variation in these hybrids, and some flies can not always with 

 certainty be distinguished from wild flies, but most of them are 

 undoubtedly distinctly darker, especiafly the wings. How this 



