Interaction in Polyhybrids 335 



different recessive genes. All seven pairs of genes interact to 

 produce a large number of phenotypes. 



The Wild-Type Drosophila. Other examples could be given 

 of the interaction of a number of genes. In Drosophila melano- 

 gaster, the various eye colors are due to nonallelic, recessive 

 genes such as white [w], ruby {rh) , vermilion {v), garnet (g), 

 carnation [car), purple (pr), sepia (se), scarlet {st) , and pink 

 [p). The normal, or wild-type, fly is red-eyed and results from 

 the interaction of the dominant alleles of all the above genes. 

 Its formula would therefore be W Rb V G Car Pr Se St P. It is 

 not literally correct to say that red eye is dominant to white. 

 The formula for white eye is wRbVGCarPrSeStP, and a 

 more correct statement is that the wild-type eye is produced by 

 a certain combination of genes, whereas the white eye is pro- 

 duced by exactly the same genes with the substitution of the 

 recessive w for the dominant w+ or W. Similarly, ruby-eyed 

 flies have the formula W rb V G Car Pr Se St P, purple-eyed flies 

 are W RbV G Car pr Se St P, and flies with pink eyes are W Rb 

 V G Car Pr Se St p. Since a wild-type and a white-eyed fly 

 differ only by the presence of W or w, it is customary to assume 

 that all the other dominants are present and to omit them from 

 the formula. In like manner, it is much simpler to state that a 

 purple-eyed fly has purple eyes because it is homozygous for pr 

 and to omit the additional requirement that all the other domi- 

 nant genes must be present. It must always be understood, 

 however, that eye color is due not to one gene alone but to the 

 interaction of many genes. 



Complementary Genes in Polyhybrids. When three or more 

 pairs of genes inj:eract, dominant and recessive epistatic, in- 

 hibiting genes and complementary and duplicate genes may be 

 involved. If three complementary genes are interacting, so that 

 all three dominants produce one character, such as color, and 

 all other combinations of genes produce a different character, 

 such as white, the F2 ratio would be 27 colored : 37 white. Four 

 such genes would give 81 colored : 175 white. 



Duplicate Genes in Polyhybrids. If three duplicate noncumu- 

 lative, dominant genes are interacting, the F2 ratio will be 63 

 dominants to one recessive, whereas four such genes will result 

 in a ratio of 255 : 1. If three pairs of duplicate, cumulative 

 genes that show dominance are present, the F2 will be in the 



