No. 576] 



VARIATION IN DROSOPHILA 



757 



A study of the F 2 generation shows that the majority 

 of the normal F 1 offspring differ from the majority of 

 the Beaded F t offspring genetically in that normals give 

 fewer Beaded offspring in the F 2 generation than do the 

 Beaded flies. 



Beaded wings showed no linkage to any sex-linked 

 character. 



Approximately one half of the flies of the F 1 generation 

 of a cross between Beaded flies and flies with characters 

 whose genes were in the second chromosome, showed 

 linkage in the following generation to second chromo- 

 some characters, while one half of the flies did not show 

 such linkage. The cases where linkage did not occur 

 gave a slightly lower percentage of Beaded offspring 

 than did those where linkage was present. An explana- 

 tion of these phenomena is sought in the suggestion that 

 there was in the second chromosome a gene, here called 1, 

 that was recessive but that in the heterozygous condition 

 intensified the dominance of another gene, called B\ 

 which was not in the second chromosome. This gene 1 

 behaves as a lethal factor preventing the development of 

 any fly that carries it in a homozygous condition. 



All of the F, offspring of the crosses of Beaded flies 

 by flies with characters caused by genes in the third 

 chromosome showed linkage in the following generation 

 between Beaded wings and the third chromosome char- 

 acters. This was taken to signify that there was in the 

 third chromosome a non-lethal gene concerned in the 

 development of Beaded wings. This gene was called B'. 

 This gene was shown to be the essential germinal factor 

 in the production of Beaded wings. It is sometimes 

 dominant and sometimes recessive. 



The determination as to whether B' should be dominant 

 or recessive seems to lie in several possibilities: 1st, the 

 nature of the egg cytoplasm; 2d, the presence or absence 

 of the gene 1; 3d, the nature of the environmental con- 

 ditions. 



With reference to environmental conditions, it was 

 shown that a larger percentage of the F, generation had 

 Beaded wings when the culture was wet than when it was 



