Xos. 620-621] SHOE TEE ARTICLES AXD DISCUSSIONS 463 



disjunction for about three months previous to this mating. 

 The semiforked females were not noticed until the bottle was 

 half through hatching, probably being overlooked. The counts 

 show that after the new character was found there were 15 such 

 females and 42 of the expected normal bristle type. This is 

 clearly a 3:1 ratio and both parents must have been heterozy- 

 gous for the semiforked gene. Although they were not brother 

 and sister, this is not improbable, because the gene seems to have 

 existed originally in the Bar stock, which, however, was not pure 

 for it. The forked female doubtless obtained the gene from the 

 Bar stock also, as her pedigree contains many Bar stock males 

 used in the non-disjunction experiments. Since attention was 

 paid only to the behavior of the X chromosomes, it is easy to see 

 that the autosomes would be interchanged from generation to 

 generation and the forked female could have a third chromo- 

 some which came originally from the Bar stock. 



The strain was kept going by brother-sister matings. One 

 F 2 culture (712) of a forked male to a heterozygous Bar forked 

 female, which was semiforked, produced all the heterozygous 

 forked females with semiforked bristles. Here both parents 

 were pure for the modifier. In Culture 722, which was an F 2 

 from 668, half of the females heterozygous for forked were semi- 

 forked. In this case, one parent was pure and the other hetero- 

 zygous for the modifier. One case was observed where a forked 

 male crossed to a semiforked female produced no semiforked 

 daughters. ■ The explanation is that both the third chromosomes 

 of the father carried the normal genes. The reverse case of 

 this was shown when a forked male was crossed to a hetero- 

 zygous Bar forked female with normal bristles. Of the hetero- 

 zygous forked females produced, approximately half were semi- 

 forked and half normal bristled. Here the father was pure for 

 the modifier but the mother was heterozygous for it and the 1 : 1 

 ratio resulted. 



Location of Modifier 

 The presence of the modifying gene in the third chromosome 

 was demonstrated by the following method, which has been used 

 before in work on Drosophila. A semiforked female was out- 

 crossed to a star dichtete male from stock. Forked star dicha?te 

 males were selected from the offspring and back-crossed to the 

 semiforked females from stock cultures. Star and dichate are 

 dominants in the second and third chromosomes respectively 



