240 THE SECOND-CHROMOSOME GROUP 



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 bi-colored (Morgan and Bridges, 1913). 



In carrying on a stock of non-disjunction by means of eosin, Bridges 

 found (July 13, 1913) that certain flies were showing an eye-color 

 considerably paler than the standard eosin. Investigation of these 

 " cream"-colored flies showed that they were double recessives, being 

 eosin plus a specific modifer of eosin eye-color. Thus, a pure stock 

 of the modifier would look precisely like a stock of wild flies. 



Shortly after the discovery of the first cream (cream a), a second 

 cream appeared (September 15, 1913) among the eosin males and 

 females of a stock culture of lethal 2. 1 Wild-type females heterozy- 

 gous for lethal 2 had been crossed to eosin miniature males, and the 

 FI wild-type daughters again crossed to eosin miniature males. The 

 mothers of the culture which gave the creams were therefore wild- 

 type females heterozygous for eosin and miniature as well as lethal 2 



( JL k +)> wn il e the fathers were eosin miniature. The cream males 

 and females which appeared were much paler than cream a, though 

 like cream a they were a light, translucent yellow with little or no 

 pinkish tinge. None of the not-eosin flies were different in color 

 from normal red flies. 



A careful examination of the stock of eosin miniature failed to show 

 any flies that did not have the standard eosin eye-color, and no lighter 

 eye-color has ever subsequently shown itself in this stock. It is evi- 

 dent that the gene for the modification had been present in the wild- 

 type flies of the lethal 2 stock, but had been unsuspected so long as 

 eosin was not present as a base. The demonstration that the cause 

 of the observed dilution of eosin was a gene behaving in inheritance 

 like the other mutant genes was easily made. 



INHERITANCE OF CREAM II. 



One of these cream males was out-crossed to a wild female. Among 

 the F 2 flies the creams reappeared, and, as in the parallel case of cream 

 a, the not-eosin flies were all indistinguishable from one another and 

 from wild flies in color. The F 2 result resembles that obtained with 

 cream a, except that, as stated, the new cream was considerably paler; 

 and it was further discovered that besides the creams, approximately 

 50 per cent of the eosin males were intermediates between eosin and 

 this cream, that is, cream II diluted eosin even in heterozygous form, 

 so that the eosin sons were visibly as well as genetically in the ratio 

 1 eosin : 2 eosin heterozygous for cream II : 1 eosin pure for cream II. 

 The entire ratio, disregarding sex, approximated 12:1:2:1, the 12 

 being the red-eyed flies. 



1 This culture was part of a generation which succeeded generation Q, table 22, p. Ill, Morgan, 

 1914, and which gave results similar to the results of generations J to Q of table 22. 



