Gene Interaction and Phenotypic Expression 



53 



one pair of genes has had an effect upon dis- 

 tinguishing the effects of the other. The 

 general term epistasis may be used in these 

 cases to describe the interference with, sup- 

 pression or masking of, the phenotypic ex- 

 pression of one pair of genes by the members 

 of a different pair of genes. Genes whose 

 detection is hampered by nonallehc genes are 

 said to be hypostatic, i.e., to exhibit hypos- 

 tasis. As dominance impHes recessiveness, 

 so epistasis impHes hypostasis. (The term 

 dominance is properly used only to describe 

 the masking by one allele of the phenotypic 

 expression of its partner, when this is a dif- 

 ferent allele.) There need be no relationship 

 between the dominance of a gene to its allele 

 and the ability of the gene to be epistatic to 

 nonallelic genes. So, in theory, epistatic 

 action may depend upon the presence of A, 

 or A', or AA' ; moreover, hypostatic reac- 

 tions may depend upon the presence of B, 

 or B' , or BB' . In view of this, it should be 

 noted that in crosses between identical di- 

 hybrids, epistasis-hypostasis can produce 

 phenotypic ratios that are still different from 

 those already described. 



Consider yet another example of a dihybrid 

 in which both pairs of genes show dominance 

 but no epistasis. Specifically, as found in 

 Drosophila, the dull red eye color of flies 

 caught in nature is due to the presence of 

 both brown and red pigments. Let A be the 

 allele which produces the red pigment and 

 A' its recessive allele which produces no 

 red pigment ; let B be another gene producing 

 the brown pigment whose allele B' fails to 

 make brown pigment. A mating between 

 two dull red dihybrid flies (from a cross of 

 pure brown by pure red) will produce off- 

 spring which are in the proportion 9 dull red 

 (containing AB) : 3 red (containing AB'B') : 3 

 brown (containing A'A'B) : 1 white (contain- 

 ing A'A'B'B'). The latter phenotypic class is 

 new to the cross, resulting, so to speak, from 

 the additive subtraction of both eye pigments. 

 This case illustrates that not only may non- 



allelic genes act upon different traits, or upon 

 the same trait in the same or a different man- 

 ner, but that their interaction may result in 

 apparently novel phenotypes. The latter inter- 

 actions do not change the number of pheno- 

 types obtained, but change their nature. 



A slightly more complex illustration of ap- 

 parently novel phenotypes comes from cross- 

 ing certain fowl (Figure 7-4). An individual, 

 from a pure line of "rose" combs is mated 

 with another individual from a pure hne of 

 "pea" combs. All the Fi show "walnut" 

 combs. Crosses of two Fi "walnut" type 

 individuals provides F2 which occur in the 

 ratio 9 "walnut" {AB) : 3 "rose" (AB'B') : 3 

 "pea" (A'A'B) : 1 "single" (A'A'B'B'). The 

 newly appearing single proves to be the 

 double recessive; pea is homozygous re- 

 cessive for one pair of genes but not the other; 

 rose is the recessive type reciprocal to pea; 

 while walnut is homozygous recessive for 

 neither pair of genes. 



In view of the present discussion, it seems 

 that any given phenotypic trait may be the 

 result of the cooperative or opposing inter- 

 action of several gene pairs acting directly, 

 and of all other gene pairs acting indirectly. 

 We are led to suppose, therefore, that the 

 total phenotype is the product of the total 

 genotype acting together with the environ- 

 ment. Two other generalizations may be 

 made. First, dominance and epistasis cause 

 phenotypic ratios to deviate from genotypic 

 ratios. The occurrence of the one and/or the 

 other results in a reduction in number of 

 phenotypic classes. It should not be implied 

 that modifications of ratios result from direct 

 effects of one gene upon another, but rather 

 that the changed ratios result from conflicting 

 gene effects at the physiological or bio- 

 chemical level. Something of the nature of 

 such conflicts can be predicted in a general 

 way from the kind of modified ratio obtained. 

 Second, in no case does a modified phenotypic 

 ratio serve to disprove the validity of either 

 segregation or independent segregation, since 



