52 



CHAPTER 7 



but act, not in the same direction, but in dif- 

 ferent, opposite, or antagonistic ways? Sup- 

 pose, now, both pairs of genes affect the same 

 trait but the effect of one gene pair is com- 

 pletely antagonistic to the detection of the 

 effect of another gene pair. An example of 

 such a unilateral opposition to phenotypic 

 expression occurs in the fruit fly, Drosophila 

 (Figure 7-3). ^ A' is a recessive allele which 

 reduces the wing to a stump while B' is a 

 recessive allele which causes the wing to be 

 curled, the dominant allele A making for 

 normal length of wing and the dominant 

 allele B for straightness of the wing. A cross 

 between two identical dihybrids does not 

 produce the customary 9:3:3:1 ratio. In 

 the present case, the ratio becomes 9 flies 

 with long, straight wings : 3 with long, curled 

 wings : 4 flies whose wings are mere stumps 

 (of which one quarter would have had curled 

 wings had the full wing formed). Here, then, 



' References to the genetics of Drosophila are given at 

 the end of this Chapter. 



a genotype of one gene pair suppresses the 

 detection of the phenotypes from another 

 gene pair. 



In other cases the two pairs of genes may 

 show mutual opposition to phenotypic ex- 

 pression. Suppose the dominant alleles A 

 and B each independently contribute some- 

 thing different but essential for the production 

 of red pigment, whereas their corresponding 

 recessive alleles A' and B' fail to make the 

 respective independent contributions to red 

 pigment production. Then crosses between 

 two identical dihybrids will produce 9 red : 7 

 nonred (composed of 3 homozygotes for A' , 

 3 homozygotes for B' , and 1 homozygote for 

 both A' and B'). Depending upon how you 

 look at it, examples of unilateral opposition 

 to phenotypic expression may also be thought 

 to involve unilateral cooperation, and cases 

 of mutual opposition to involve mutual 

 cooperation. 



In all cases where two pairs of genes affect 

 the same trait, whether they interact pheno- 

 typically by superposition or by antagonism, 



FIGURE 7-3. Drosophila melanogaster mutants 

 showing the no-wing (left) and the curled wing 

 {right) phenotypes. {Drawn by E. M. Wallace.) 



