250 



THE SECOND-CHROMOSOME GROUP 



dichsete. It was now realized that the back-cross tests of females 

 heterozygous for pinkish and black had not excluded the possibility 

 of pinkish being in the second chromosome, though they had shown 

 that, if so, it could be only in one or the other end-region and not near 

 black. Accordingly, exactly the same procedure was followed as in 

 the tests for the location of cream b, that is, a pinkish female was out- 

 crossed to a male which had the second-chromosome dominant star 

 as well as dichsete. The FI eosin star dichsete males were then back- 

 crossed to pinkish females. The result showed (table 96) that the 

 gene for pinkish is in the second and not the third chromosome; for, 

 as well as could be judged, none of the star flies were pinkish, while 

 all the not-stars seemed to be pinkish, and also dichsete was present 

 in half of both the star and the pinkish classes. 



TABLE 97. B. C. offspring given by a star female from 

 table 96, when back-crossed to a pinkish male. 



TABLE 98. F z offspring given by the FI wild-type females and 

 eosin males, from out-cross of pinkish females to wild males. 



LOCUS OF PINKISH. 



In the light of this test, and from the fact that there was about 50 

 per cent of crossing-over between black and pinkish, we could place 

 pinkish in either the extreme left or the extreme right end-region of 

 the second chromosome. Fortunately, one advantage of the test just 

 described is that it left us in possession of females heterozygous for 

 star and for pinkish, and a back-cross test showed (table 97) that 

 there is very free crossing-over between star and pinkish. Pinkish 

 is known, therefore, to be in the right-hand end of the second chromo- 

 some, in the neighborhood of arc, speck, balloon, etc. Had the test 

 given almost no crossing-over between star and pinkish, we should 



