Inhibiting Genes 325 



Inhibiting Genes 



Since the dominant gene W in squash is epistatic over Y, W 

 and w produce two different phenotypes in the presence of either 

 Y- or yy genotypes, but the Y and y genes produce two dif- 

 ferent phenotypes only when the other recessive, w, is homozy- 

 gous. The recessive gene a in rodents is epistatic to C and c so 

 that the latter genes produce two different phenotypes in the 

 presence of either A or a, whereas genes A and a produce two 

 phenotypes only in the presence of the other dominant, C. In 

 some organisms two genes, A and a, produce two different pheno- 

 types in the presence of B but produce only one when b is homo- 

 zygous, whereas genes B and b produce two phenotypes only in 

 the presence of homozygous a and only one when A is present. 

 Such cases are classically called inhibiting factors, but this situ- 

 ation has been appropriately called dominant and recessive epis- 

 tasis by Snyder. 



The classical example of this type of gene interaction is found 

 in poultry. If white Leghorns are crossed with white Plymouth 

 Rocks, the Fi is white, but the Fo segregates into 13 white : 3 

 colored. Two pairs of genes interact to produce color. Gene C 

 is dominant over c, and / is dominant over i. Genes C and i 

 together produce a colored fowl, but the combinations CI, cl, 

 and ci are white. Gene C is thought to be a dominant gene 

 which would produce color, but / is regarded as a gene that in- 

 hibits the action of the gene product of C. White Leghorns 

 are homozygous for C and would be colored except for the fact 

 that they are also homozygous for /, which inhibits C from pro- 

 ducing color. White Plymouth Rocks are cc ii and are white 

 because they lack any gene that can produce color. A cross of 

 these two breeds produces the following results: 



White Plymouth Rock X White Leghorn 

 cc ii CC II 



Fi: Cc Ii = white because of inhibiting gene 



^3/ — 9CI = white because of inhibiting gene 



3C<^ 



"Ii — dCi = colored because of no inhibition 

 F2: 



•3/ — 3c/ = white because of no color-producing gene 

 lc\^ and a color inhibitor 



^li — lei = white because of no color-producing gene 



