52 



CHAPTER 4 



said to be hypostatic, or to exhibit hypo- 

 stasis. As dominance implies recessiveness, 



so epistasis implies hypostasis. There need 

 be no relationship between the dominance 

 o\ a gene to its allele and the ability of the 

 gene to be epistatic to nonalleles. In theory. 

 then, epistatic action may depend upon the 

 presence of A. A\ or A A'; moreover, hypo- 

 static reactions may depend upon the pres- 

 ence of B, B\ or BB'. Consequently, it 

 should be noted that in crosses between 

 identical dihybrids. epistasis-hypostasis can 

 produce phenotypic ratios still different from 

 those already described. 



Consider another example of a dihybrid 

 in which both pairs of genes show domi- 

 nance but no epistasis. In Drosophila, the 

 dull-red eye color of flies found in nature 

 is due to the presence of both red and brown 

 pigments. Let A be the allele which pro- 

 duces the red pigment and A' its recessive 

 allele which produces no red pigment; let B 

 be the nonallele producing the brown pig- 

 ment whose allele B' makes no brown pig- 

 ment. A mating between two dull-red dihy- 

 brid flies (from a cross of pure red, A A B'B' 

 by pure brown. A' A' BB) produces offspring 

 in the proportion 9 dull-red (containing 

 A — B — ):3 red (containing A — B'B'): 3 

 brown (containing A' A' B — ) : 1 white (A' A' 

 B'B'). The last phenotypic class, resulting 

 from the absence of both eye pigments, is 

 new in this series of crosses. This case illus- 

 trates that the interaction of nonallelic genes 



may result in apparently novel phenotypes. 

 Such interactions change not the number but 

 the kind of phenotypes obtained. 



The preceding discussion suggests that any 

 given phenotypic trait may be the result of 

 the interaction of several gene pairs. One 

 is even led to conclude that the total pheno- 

 type is the product of the total genotype 

 acting together with the environment. The 

 difference between phenotypic and geno- 

 typic ratios is often due to products of gene 

 action — by alleles and nonalleles — which 

 superpose, cooperate, or conflict at the phys- 

 iological or biochemical level. It is also pos- 

 sible that there is sometimes a direct in- 

 fluence of one gene upon the ability of an 

 allele or nonallele to act. Although the na- 

 ture of gene interactions can be predicted 

 partially, in a general way. from the kind 

 of modified ratio obtained, an understanding 

 of the mechanisms involved must ultimately 

 be based upon a knowledge of how genes 

 act and the nature and fate of gene products. 

 In no case has a phenotypic ratio that differs 

 from the expected genotypic one served to 

 disprove either segregation or independent 

 segregation. In fact, segregation and inde- 

 pendent segregation were first proved despite 

 the misleading phenotypic simplifications of 

 genotypic ratios wrought by the occurrence 

 of dominance; moreover, the principle of 

 independent segregation could also have been 

 first proved from crosses involving epistasis 

 or apparently novel phenotypes. 



SUMMARY AND CONCLUSIONS 



When two different traits were studied separately, the phenotypic alternatives were 

 found to be due to the presence of a single pair of genes in each case. Studies were 

 then made of the distribution of phenotypes in successive generations when these two 

 pairs of traits were followed simultaneously in the same individuals. The data ob- 

 tained showed that each trait is due to the presence of a different pair of genes, proving 

 that the genetic material is made up not of a single segregating pair of genes but of a 

 number of segregating gene pairs. Moreover, the results are best explained by the 

 principle that the segregation of one pair of alleles is at random with respect to the 

 segregation of all the other nonalleles tested. The simplest hypothesis for the physical 



