570 GENETICS AND EVOLUTION 



P, 



Gomefes 



White 



Gometes 



Purple 

 00110 



Purple 



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Phenotypes! 9 purple = 7 white 



Figure 33.1. Diagram of a cross illustrating the action of complementary genes, 

 the two pairs of genes which regulate flower color in sweet peas. At least one C gene 

 and one E gene must be present to produce a colored flower. The absence of either 

 one or both results in a white flower. (Villee: Biology.) 



In the course of breeding experiments with varieties of cultivated 

 sweet peas, Bateson and Piinnett tound that purple flower color was 

 dominant to white. Several different varieties with white flowers are 

 known and the mating of most white-flowered plants produces only 

 white-flowered offspring. However, when plants from two particular 

 white-flowered varieties were crossed, all the offspring had purple 

 flowers! When two of these purple Fi plants were crossed, or when they 

 were self-fertilized, an t\, generation was produced in the ratio of 9 



