OF MUTANT CHARACTERS. 



217 



stock?) or whether there were two independent mutations is uncertain. 

 Both occurrences have contributed to similar "epidemics of mutation' ' 

 in the cases of purple, jaunty, arc, etc. 



CHROMOSOME CARRYING DACHS. 



Since it had become apparent while pure stock was being extracted 

 that dachs was recessive and not sex-linked, we proceeded directlj' to 

 tests of its autosome group. A dachs male was out-crossed to a black 

 female, and from a pair of the wild-type Fi flies an F2 culture was 

 raised (table 60) . 



Table 60. — Pi, dachs cf X black 9 ■ Fi wild-type cf X /^i wild-type 9 . 



The F2 flies were in the 2:1:1:0 ratio, which had become recog- 

 nized as the typical result for two recessives in the same autosome 

 crossed to each other and carried to F2 ("repulsion" F2). This ratio 

 is the necessary result of the absence of crossing-over in the male and 

 is independent of the amount of crossing-over in the female. 



A similar cross of dachs by curved likewise gave no double recessives 

 in F2 (table 61). The reason in this case for crossing to two different 

 recessives in the same chromosome was to locate dachs more speedily 



Table 61.— Pi, dachs 9 9 X curved cfd^. Pi wild-type 9 9 + Pi 



wild-type cf cf. 



by working out simultaneously two cross-over values. However, the 

 dachs curved cross was not carried to the back-cross stage, because 

 at that time the locus of curved was itself not sufficiently well estab- 

 lished and secondarily because dachs seemed poorly viable in the 

 dachs curved F3. 



From the dachs X black F2, dachs and blacks were crossed en 7nasse, 

 and in F3, a few dachs black double recessive were secured. One of 

 these was crossed to a wild male as the Pi for the j^roposed crosses, 

 and the rest were mated together to supply a dachs black stock to be 

 used in the back-cross tests. 



