328 



Interaction of Genes 



(white) pigment which could be converted into purple by the 

 colorless enzyme, P. Thus Cp plants would be white because 

 there was no enzyme present to convert the white pigment into 

 purple, whereas the cP and cp plants would be white because 

 there was no pigment present. 



These complementary genes can be considered as an example 

 of epistasis in which two independent recessive genes are epi- 

 static to both alleles of the other pair. Thus in the presence 

 of cc, neither the P nor the p genes show a different expression, 

 whereas in the presence of pp, both C and c produce the same 



CP 



Cp 



cP 



cp 



CP 



Cp 

 cP 

 cp 



Fig. 92. Checkerboard showing the 9 : 7 ratio in sweet peas. A cross 

 between two white strains genotypically CC pp and cc PP produces a 

 purple Fi, CcPp, because these genes are complementary, and an F2 segre- 

 gating into 9 purple (C P) : 7 white (3Cp + 3cP + lcp). (Based on the 

 work of Bateson.) 



phenotypes. Gene c is therefore epistatic to P and p, and p is 

 epistatic to C and c when c and p are homozygous. In the 

 heterozygote CcPp the recessive epistasis of c and p is lost be- 

 cause neither is homozygous. Examples of this and other ratios 

 in maize are found in Fig. 93. 



Blakeslee's study of complementary genes in the yellow daisy, 

 Rudbeckia hirta, showed that the Cp and cP classes could be 

 distinguished in this plant by chemical tests. He had two yellow- 

 coned strains which were alike phenotypically but different geno- 

 typically. AVhen they were crossed, the Fi plants had purple 

 cones and the F2 segregated into %6 purple-coned and %6 

 yellow^-coned plants. This ratio is clearly one of complementary 

 genes, in which the AB type is purple-coned. Can it be explained 

 by the pigment-enzyme theory? If A produced a yellow sub- 

 stance which turned purple in the presence of the gene product 

 of B, and if a produced a yellow substance like the other in 

 color but different chemically because it did not react with B, 



