Modifying Genes 337 



spots on the coat, no matter how many modifying genes are 

 present. 



Another excellent example is the modifiers of eosin eye in 

 Drosophila melanog aster. Eosin is one of the series of multiple 

 alleles at the white locus, and eosin-eyed flies are w^w^ (plus all 

 the dominants mentioned previously). The shade of the eosin 

 color may vary from light to dark, depending upon the presence 

 of several modifying genes. If a nonallelic gene called dai^k is 

 present, the eosin color is darker than in the absence of the 

 dark gene. There are seven genes that have a lightening effect 

 on the eosin, and some lighten the color much more than others. 

 Pinkish lightens the color slightly, and the gene known as whit- 

 ing lightens it so much that it is almost white. Intermediate 

 shades are produced by five genes known as cream c, cream b, 

 cream a, cream ///, and cream //. They are all modifying genes, 

 because their only effect is to vary slightly the expression of a 

 character determined fundamentally by another gene. 



An interesting and rather unusual group of modifying genes 

 affects the frequency with which another gene mutates. In 

 Drosophila virilis, a species closely related to the more familiar 

 melanogaster, is a gene known as ''miniature-gamma" {mt-y) ^ 

 which produces a miniature wing instead of one of normal size. 

 This gene, however, has a peculiar property of suddenly mutat- 

 ing back to the wild-type or normal condition. In the develop- 

 ing wing of a miniature-gamma fly, all the cells naturally are 

 homozygous for mt-y, but here and there the gene in one of the 

 cells of the wing will mutate. All the cells that develop from 

 these cells will be normal. Thus the wing will be a mosaic of 

 miniature and normal tissue. This is the effect of the "main" 

 gene, and the modifying genes stimulate the rate of this reverse 

 mutation greatly. Two of these modifying genes are dominants, 

 S-1 and S-3; the other, s-2, is a recessive. A miniature fly homo- 

 zygous for S-2 was crossed with an s-2 s-2 miniature. All the 

 Fo flies were miniature, but one-quarter were homozygous for 

 s-2. Out of 796 F2 flies, Demerec found that 584 showed little 

 or no change of mt-y to the dominant allele and therefore little 

 or no mosaicism in the wing, while the other 212 flies had mosaic 

 wings showing that mosaic formation is greatly stimulated by 

 genes s-2. Genes *S-1, s-2, and *S-3 are definitely modifying genes 

 and produce no effect except in mt-y mt-y flies. 



