166 



THE SECOND-CHROMOSOME GROUP 



wide angle at which it is held, the curved wing seldom becomes bedrag- 

 gled, and the flies are very free from the tendency to become stuck in 

 the food or to be drowned. As far as we have observed, the wings are 

 the only part affected by the curved mutation. 



CHROMOSOME OF CURVED. 



The first hnkage in Drosophila that did not involve the X chromo- 

 some was thut observed by Bridges (March 1912) between black and 

 curved (Bridges and Sturtevant, 1914). In the F2 generation of a 

 cross of black to curved no black-curved flies appeared. This case 

 was interpreted as one in which the hnkage was of such a strong 

 order that no crossing-over had taken place. On the basis of this 

 linkage it was concluded tha,t curved was in the same chromosome as 

 black, that is, in a "second chromosome." A systematic search for 

 hnkage between the other known non-sex-linked genes in Drosophila 

 was undertaken, and a similar relation, "repulsion," was studied in 

 the case of black and vestigial (Morgan and Lynch, 1912). Further 

 work by Morgan in the case of black and vestigial showed that the 

 non-appearance of the double recessive, where two second-chromo- 

 some recessives entered the Fi from opposite parents, could be ex- 

 plained on the basis of lack of crossing-over in the male. It was soon 

 shown that this same explanation applied in the case of black curved, 

 and that in the female there is considerable crossing-over between these 

 two loci. 



LOCUS OF CURVED. 



In order to obtain stock of the double recessive, the black-curved 

 cross was repeated, and some of the F2 blacks were mated in mass- 

 cultures to the F2 curved flies of the other sex. If crossing-over were 



taking place in the Fi female ( —r > | ) , 



F2 blacks should be heterozygous for curved ( -r ) and a corre 



sponding few of the curves should be heterozygous for black ( 



, a few of these 



\ c 



y 



The appearance in F3 of a few black flies (-r — —j showed that at 



least one of the F2 curved flies had been the result of crossing-over. 

 By inbreeding these F3 blacks, black curved flies (25 per cent) were 

 secured in F4. 



That the absence of black curved flies in F2 was really due to lack of 

 crossing-over in the male was shown by the results of the back-cross 

 tests carried out upon double heterozygous males as compared with 

 like tests of the females. In the tests of the males no cross-overs 

 appeared in a total of 1,066 flies, while in the tests of the females 1,717 



