Unit Characters and Gene Interaction 25 



were not unit characters but were due to the interaction of two 

 pairs of genes. 



The situation is not even so simple as we have just pictured it. 

 The wild-type wing is due not only to Vg and M but also to the 

 alleles of the genes that produce cut wing, jammed wing, curved 

 wing, plexus wing, curled wing, bent wing, and other wing vari- 

 ations. A partial formula for the wild-type wing, then, would be 

 CtCtMM jjVgVgCC PxPxCuCuBtBt; a miniature fly would 

 be CtCt mm ijVgVg CC PxPxCuCuBtBt] and a vestigial fly 

 would be CtCtMM jjvgvgCC PxPxCuCuBtBt. 



This example shows that the wild-type fly has a certain com- 

 bination of genes. It shows further that the miniature fly dif- 

 fers from the wild-type in one certain pair whereas the vestigial 

 differs from the wild-type with respect to a different pair. Ordi- 

 narily, in discussing miniature versus wild-type flies, we do not 

 bother to write the full formula in either case but only the 

 differential, which is M and m; we merely understand and imply 

 that the other genes are present and are alike in each case. When 

 we say that a vestigial fly is vgvg we recognize that these other 

 genes are present but we omit them from the formula because 

 they are the same in both the vestigial and wild-type flies. It 

 is incorrect to say that Vg produces wild-type flies and vg pro- 

 duces vestigial, but it is permissible as a time and space saver 

 provided we realize that a number of other genes which affect 

 the wing are also present and that they are the same in 

 each case. 



This example of gene interaction is further interesting be- 

 cause it shows that the wild-type fly does not necessarily consist 

 of all dominant genes. At the locus of jammed, the wild-type 

 fly has the recessive gene, whereas the nonwild-type, jammed, is 

 produced by the dominant allele. The wild-type fly has a certain 

 combination of genes. Each other type has a somewhat dif- 

 ferent combination. Throughout the course of evolution, flies 

 with the wild-type wing apparently were better adapted to their 

 environment than the other types. Because of this evolutionary 

 factor, a certain combination of genes is found much more fre- 

 quently in nature than any other combination. We frequently 

 think of this more frequent combination as the normal one 

 because it is the one present in almost all the flies we gather in 



