156 



THE SECOND-CHROMOSOME GROUP 



gave only wild-type sons and daughters, from which it was concluded 

 that the character was recessive and non-sex-linked. The F 2 genera- 

 tion gave only a few blistered (about 1 in 10), and these were nearly 

 all females. By mating together the blistered individuals hi pairs a 

 stock was obtained which must have been genetically homozygous for 

 blistered, although only about half of the females and about a quarter 

 of the males showed the character. It had been noticed that the size 

 of the vesicle varied from a very minute bubble to one which covered 

 over half the area of the wing, and that there was not a very close 

 correspondence between the two wings; frequently the blister would 

 appear in only one of the two wings. A closer inspection showed that 

 the wings which did not show a vesicle had a small plexus of veins 

 in the region occupied by the vesicle (figs. 74 a and 6), and it was found 

 that the flies could be quite readily classified by this character, irre- 

 spective of whether they showed the blister or not. At the same 

 tune the results given by breeding from these abnormally veined flies 

 showed that the venation and the blistering were both products of 

 the same gene. A third manifestation of this gene is a sharp bend in 

 the distal end of the fourth longitudinal vein (shown hi text-figure 74, a). 



TABLE 18. PI mating, curved cf X blistered 9 ; F\ mating, 

 wild-type 99 + wild-type a" cf . 



THE CHROMOSOME OF BLISTERED. 



Little was done with blistered, aside from getting the pure stock 

 just described, until the discovery that black and curved were hi the 

 same chromosome gave a sharp impetus to further study of autosomal 

 linkage. Shortly thereafter (April 3, 1912) blistered was crossed to 

 curved and three F 2 cultures were raised (table 18). No curved- 

 blistered flies were found hi the F 2 . The numbers given in table 18 

 represent only the first counts from each of these three cultures, and for 

 this reason the number of wild-type flies, which are the first to hatch, 

 is abnormally high, No further records were made of the F 2 offspring, 

 because of the suspicion that blistered might not be distinguishable 

 in curved flies, and that the absence of the double recessive might be 

 due to inhibition or masking instead of the supposed linkage. Never- 

 theless, all the F 2 flies were examined in the hope of finding a double 



