276 



THE SECOND-CHROMOSOME GROUP 



In the back-crosses the same two kinds of Fi flies should occur 



and ), which were both tested by back-crossing 



b VaU b vj h 



VgU Vg/ 5 



with males of the constitution t 



VgH 



Table 124. — Pi. black nick cf 



/b_vl\ 

 [b rj 



X vestigial cf. 



These two types of back-crosses should be in equal numbers, and 

 3 of the first type and 2 of the second were found. 



If the above were the true explanation of the history of the nick 

 crosses, then whenever nick is out-crossed to vestigial half of the 



7/ 



offspring should be vestigial - and half nick 



This test was 



applied and found to hold in part (table 124, cultures 2464 and 2465), 

 for while half the flies were vestigial the remainder were not all nick 

 as expected, but 141 were nick to 106 that were wild-type. 



It was assumed that not all the compounds showed nick because of 

 overlap, which might well be the case as far as the agreement with 

 previous results went; for the nicks had quite uniformly failed to be 

 as numerous as expected. 



If the nick character is the result of a vestigial-nick compound, then 

 it is more efficient in testing the linkage to back-cross by a black 



vestigial male than by a black nick male ( 9 Xbv d^ ), for in 



\ b Vg / 



this case twice as many nicks should appear, as though the father were 

 himself nick. Several such tests were started, but all failed to breed 

 except one, which happened to have come from the black vestigial 

 second chromosome of the nick parent (culture 2461, table 123). 



A stock of flies homozygous for the nick allelomorph should be 

 obtainable by the paradoxical method of selecting against the nick as 

 well as the vestigial flies that should appear on inbreeding the flies 

 showing the nick character. 



