Epistasis of a Dominant Gene 321 



on whether V or v is present; and the phenotypes produced by 



V or V will depend upon whether the plant also has S or s. The 

 inheritance of the ''rose," "pea," and "walnut" combs in fowl is 

 of a similar nature, depending upon the interaction of two pairs 

 of nonallelic genes. 



Epistasis of a Dominant Gene 



Two nonallelic genes do not always cooperate equally in the 

 production of a different character, for sometimes one of them 

 has so strong a reaction that it prevails over the other. Let us 

 assume that gene A is dominant over a and that B is dominant 

 over b. It is conceivable that the product of gene A could be 

 so potent that it would prevail over the products of both B and 

 b and make the AB and Ab phenotypes identical. Such reactions 

 do occur and are probably of the same nature as the simple 

 dominance of one allele over the other. The term "dominance," 

 however, is best reserved for the relation between two alleles. 

 When one gene exerts a dominating influence over another gene 

 which is not an allele, the first gene is said to be epistatic and 

 the gene which is suppressed is hypostatic. The phenomenon is 

 called epistasis. 



An excellent example of the epistasis of a dominant gene is 

 found in the summer squash. Sinnott and Durham showed that 

 two pairs of genes for fruit color were interacting in an epistatic- 

 hypostatic relationship. Gene W is dominant to w and is epi- 

 static to the Y and y genes. A plant with one or two W genes 

 has white fruit. The other genes, Y and y, exert their effect 

 only in ww plants. Plants which are ww and have one or two 



Y genes have yellow fruit, whereas ww yy plants have green 

 fruit. If a white-fruited plant homozygous for W and for the 

 hypostatic dominant gene Y is crossed with a green plant, the 

 Fi is Ww Yy and has white fruit. The F2 segregates as follows: 



37 _^ QWY 



^ly -> 3Wy 



= 12 white 



.3F -* SwY = 3yeUow 



Iw 



'I2/ — > Iwy = 1 green 



