286 



THE SECOND-CHROMOSOME GROUP 



The one back-cross culture that produced results (4044, table 130) 

 showed that squat is not far from black, there being 9 cross-overs in a 

 total of 82 flies, or 11.0 per cent. 



The pair of complementary classes with the smallest sum is squat 

 black and plexus, which are therefore probably the double cross-overs. 

 If this is true, then squat lies to the left of black, a position which agrees 

 with the amount of its crossing-over with star and also with the high 

 value (47.6) for squat-plexus. 



TABLE 130. PI, squat 9 X black plexus cf ; B.C., F\ squat 9 

 back crossed by black plexus cf . 



Because of the small number of flies, this determination is not very 

 accurate, but since the character is so poor it was not thought best to 

 do any more then, and the problem may never be taken up again. 



A stock of squat was made up and has since been running with- 

 out selection. A recent (February 1918) examination of the stock 

 showed only an occasional squat, one of which was drawn (figure 85). 

 The original stock was not pure and the present scarcity of squats 

 may be due to their poor viability in competition with the non-squat 

 sibs, to a possible lethal nature of the squat homozygote, and to some 

 extent to " eclipse" of the squat character. 



LETHAL I la SAME AS (l lla ). 

 ORIGIN OF LETHAL lla. 



In looking over the star black curved stock (December 4, 1915) in 

 search of a virgin black curved female, Bridges noticed that one of the 

 black curved males had an eye-color much like purple. This male was 

 out-crossed to a wild female, and several pairs of the wild-type FI 

 flies were bred for the F 2 generation. It was suspected that the eye- 

 color, called crimson, was sex-linked, and this was confirmed by the 

 fact that crimson reappeared only in the F 2 males, where it constituted 

 about half the flies. As soon as the sex-linkage of crimson was estab- 

 lished the counts on the F 2 cultures were stopped and the cultures were 

 thrown away in favor of a back-cross culture, which had been made 

 by mating the original crimson male to one of his wild-type daughters. 

 This back-cross culture was continued, since it gave crimson females as 

 well as crimson males and a stock was directly obtainable. As a side 

 issue it was decided to make counts on this back-cross culture to illus- 

 trate the independence of the new mutant crimson and the second- 

 chromosome characters. In making the counts only crimson and 



