No. 576] 



VARIATION IX DROSOPHILA 



733 



strongest with the character Arc and weakest with Black. 

 If there is a gene in the second chromosome which aids in 

 producing Beaded wings, it seems probable that it is 

 located nearer to Arc than to Black and on the side of 

 Arc away from Black. 



TABLE XIV 



It remains to consider Type 1 7 and to find the reason 

 for the existence in the F\ generation of flies whose off- 

 spring show no linkage between Beadedness and second 

 chromosome characters, and in the same brood, flies whose 

 offspring do show such linkage. The most obvious " ex- 

 planation" would be, of course, that the factor in the 



TABLE XV 



second chromosome was a "lethal" such as the factor 1, 

 described in an earlier section of this paper. If this were 

 the case, there should be some flies in the Beaded stock 

 homozygous for L, the normal allelomorph of this gene, 

 and also for B', i. e., B'LB'L. These flies should have 

 fewer Beaded offspring than those heterozygous for 1, 

 and none of these F t offspring should give linkage with 

 second chromosome characters. As a matter of fact, in 

 the F 2 results given in Table XVI for Beaded by Black, 

 no linkage was observed; but this case is not good evi- 

 dence, for it was made in the first attempts to solve the 

 problem of Beaded wings, and I had not yet learned the 



