274 



THE SECOND-CHROMOSOME GROUP 



appear much like the various types of strap, except that the wings 



do not diverge. There may also be other excisions at the outer edge 



of the tip or along either margin, rarely in the outer margin, often in 



the inner. 



CHROMOSOME CARRYING NICK. 



One of the black nick males was out-crossed to a wild female and 

 two Fo pair cultures raised. One of these (2190) repeated the result 

 of the first F2, but the other (2191) gave no nick whatever. In place 

 of nick there was present vestigial. Some of these black vestigial 

 flies were crossed to black vesti- 

 gial flies of stock and gave only 

 black vestigial Fi offspring. Others 

 mated together likewise gave only 

 black vestigial offspring. The orig- 

 inal male must, then, have carried 

 both black and vestigial in one of 

 its second chromosomes. 



The fact that most of the F2 nick 

 flies were also black seemed to 

 show that black and nick were in 

 the same chromosome. But that 

 this black-bearing chromosome was 

 not the one carrying vestigial 

 seemed no less clear from the fact 

 that no vestigials had appeared in 

 the F2 with the black. This reason- 

 ing leads to the supposition that 

 the original male, noted as dark, 

 was really homozygous black, which 



is not impossible, provided the weakness of the color were due to 

 age or that the male had come from a crowded or poorly fed culture. 



LOCUS OF NICK. 



The character nick had shown a decided linkage with black, where- 

 fore its gene was known to be in the second chromosome. To determine 

 its locus a back-cross was started by mating a black nick male to a star 

 female and testing the Fi star females by black nick males (table 123). 

 Two of the back-cross cultures (2327, 2329) gave nick; but instead of 

 the nick being 50 per cent of the flies as expected from a back-cross, 

 it was only 24.2 per cent. Correspondingly there was a superabun- 

 dance of black not-nick flies, so that some condition for the devel- 

 opment of the nick character was absent from many of the flies. A 

 calculation based on the nick flies showed that the apparent locus of 

 nick was to the right of black and 19 units distant. This was close to the 

 locus of vestigial and suggested that there might be some relation 



Text-figure 84. — Vestigial-nick compound 

 showing a slight development of the nick. 

 More extreme forms are scarcely to be dis- 

 tinguished superficially from "short" 

 notches or from broad straps. 



