338 CALVIN B. BRIDGES 



the result of the coaction of a specific modifying gene (cream a, 

 b, etc.) and of a particular gene (eosin) which serves as a base or 

 chief factor. The scale of the modifications of eosin produced by 

 these several modifiers ranges on the one hand to a deep pink 

 darker than eosin, and on the other hand to a pure white, without 

 considering the effects of combinations of two or more modifiers. 

 In origin these modifiers were entirely independent of one an- 

 other, and the order of their occurrence bears only a random 

 relationship to the dark-light seriation. By combination of 

 several of these simple modifiers a multiple heterozygous stock 

 could be obtained which would be amenable to selection and 

 which would offer upon analysis a satisfactory parallel to such a 

 case as that shown by Castle's hooded rats. 



CREAM A 



A pure stock of the sex-linked eye-color eosin shows a strong 

 sexual bicolorism, that is, the eye color of the eosin female is a 

 rather deep pink, only slightly yellowish, while the eye color of 

 the eosin male is a pinkish yellow, much lighter in tone than the 

 color of the female (see Morgan, '12, for an account of the origin 

 of eosin and a colored plate showing this difference in color). 

 Eosin females and males maintain this constant difference in color 

 wherever they reappear after crossing, and all double recessives 

 involving eosin, for example, eosin vermilion and eosin pink, are 

 likewise bicolored (Morgan and Bridges, '13). Eosin is allelo- 

 morphic to white — an eosin female mated to a white male gives 

 in Fi no wild-type (red-eyed) daughters, but only daughters 

 (white-eosin^ compounds) which are intermediate in color be- 

 tween white and eosin. The sons of this cross are of the regular 

 light color, which happens to be the same as that of the white- 

 eosin female. 



In carrying on the stock of non-disjunction, such a mating as 

 that just described had been repeated through several generations 

 when it was noticed (July 15, 1913) that the flies in one of these 



^ We denote compounds by the use of the hyphen and do not use the hyphen for 

 non-allelomorphie multiples, for example, white-eosin, but eosin vermilion. 



