OF MUTANT CHARACTERS. 



293 



made. In the next generation, however, two cultures produced abun- 

 dant offspring (table 137) . Crossing-over between star and telescope 

 was very free, there being 44.4 per cent observed crossing-over in 

 a total of 531 flies. By comparison of this value with the other star 

 cross-over values it seems likely that telescope is to the right of black, 

 probably to the right of purple, and most probably in the neighborhood 

 of vestigial. A position at 66.5 units to the right of star is given by a 

 correction of the cross-over value, according to the probable coinci- 

 dence of 100. 



TABLE 137. PI, telescope 9 9 X star &<?;B. C., FI star 9 X telescope 



Time was liieri lacking for testing this location further. If the locus 

 should be found to be to the right of curved the mutant would be 

 valuable for some purposes, since the character is fairly easy of classi- 

 fication and the sterility seems less pronounced. Pending further 

 tests, the stock was rearranged so that the danger of loss through 

 sterility was eliminated and also the simple stocks of purple and tele- 

 scope were replaced by a single stock. Telescope males (from 4634) 

 were out-crossed to purple females of pure stock, and the FI wild- type 

 males were again out-crossed to purple females. In the following 

 generation, because of no crossing-over in the male, only two classes 



were produced, purple and wild-type ( , ^ ). By mating the wild- 



\Pr t 3 / 



type males to the purple females, which do not need to be virgin, the 

 stock is renewed each generation. 



SECOND-CHROMOSOME "MODIFIERS' 

 BRISTLE-NUMBER. 



FOR DIOiCTE 



Families of dichsetes that differ in mean bristle-number have been 

 established by Sturtevant through selection (August 3, 1916). That 

 these differences are in part due to one or more second-chromosome 

 modifying genes has been shown by the following method: 



A dichsete of a selected line was crossed to speck of a stock that had 

 been long and closely inbred in order that it might become homozygous. 

 Dichsete being dominant, half of the offspring were dichaete, and all of 

 them were heterozygous for speck. The FI dichaete males were then 

 back-crossed to speck females. They produced dichaete offspring that 

 were not-speck and as many others that were speck ; from their father 



