OF MUTANT CHARACTERS. 



CHROMOSOME CARRYING STRAP. 



201 



The view of strap prevalent at that time was that it was vesti^nal 

 modified in the direction of the wild form by a recessive autosomal 

 modifier. On this assumption it was expected that in crosses to wild 

 there would be produced in F2 vestigials as well as straps, and also 

 some flies having the modifier only. It was problematical wlmt 

 these latter flies should look like, though in general it was expected 

 that they might have longer wings than normal, just as strap wiis 

 larger than vestigial. In F2, however, no vestigial appeared and the 

 only certain classes were the wild-type and the strap flies in the usual 

 3 : 1 ratio. It is true that a few of the flies were vestigial-like, but none 

 were typical vestigials, and these intermediate forms were regarded as 

 fluctuations of strap. 



One of these vestigial-like straps was crossed out to wild and 8 F2 

 cultures were raised. Among the 940 F2 ofifspring there were only 20 

 that repeated the vestigial-like appearance ; the remainder (175) of the 

 strap flies graded from this shortest type through the usual range of strap. 



The F2 (table 46) from the cross of strap to black gave a typical 

 2:1:1:0 ratio, which showed that strap was in the second chromosome, 

 as had been concluded from the failure of vestigials to appear in the 

 former F2. There was here also no apparent crossing-over between 

 vestigial and a possible vestigial modifier responsible for strap. 



Table 46. — Pi, strap 9 X hlack cT; Fi wild-type 9 -\- Fi wild-type cf . 



It appeared, then, either that strap is vestigial plus a recessive 

 modifier whose locus is very close indeed to that of vestigial, so tliat no 

 crossing-over was detected in the very extensive F2 counts, or, more 

 probably, that strap is an allelomorph of vestigial and recessive to both 

 vestigial and the wild-type allelomorph. 



Quite recently several rather complicated experiments have been 

 attempted in an effort to distinguish between these two alternatives, 

 but with no decisive result. We believe, however, that the whole 

 trend of the evidence in Drosophila is to the effect that tyj)es that 

 behave like strap and vestigial are examples of multiple allel()m()ri)hism 

 and not of close hnkage. It is probable that the vestigial system 



