326 Interaction of Genes 



A 13 : 3 ratio is obviously a case of a 9 : 3 : 3 : 1 ratio in 

 which three of the terms appear alike. Independently, the / 

 and i genes give a 3 : 1 ratio in the presence of C but not in 

 cc individuals. Thus CC II X CC ii gives a white Fi [CC li) 

 and an F2 ratio of 3 white {CI) : 1 colored [Ci) ; but cell X 

 CC ii gives a white Fi (cc Ii) and all white in the F2 (3c/ + lei). 

 Similarly, the C and c genes give a 3 : 1 ratio in ii fowl but not 

 when gene / is present. An inhibiting gene is generally regarded 

 as a gene that has no effect of its own but can act only to inhibit 

 a nonallelic gene. In this sense, I and i produce no phenotypic 

 effect unless C is present and therefore produce two different 

 phenotypes only in fowl that also have the gene C. Similar in- 

 hibiting genes have been identified in maize (Fig. 91) and other 

 organisms. 



Complementary Genes 



The term "complementary factors" has been applied to any 

 two nonallelic genes that act together to produce a phenotype 

 different from that produced by either alone. The term as 

 originally proposed, however, was restricted to a case in which 

 two nonallelic dominants produced one phenotype, whereas the 

 two recessives or either dominant with the other recessive pro- 

 duced a second phenotype. Bateson in 1905 discovered that if 

 he crossed two white-flowered strains of the Emily Henderson 

 sweet pea, purple-flowered plants were produced in the Fi. The 

 two strains were phenotypically identical in every respect but 

 they must have been different genotypically as, otherwise, the 

 Fi would have been white-flowered. When the F2 generation 

 was raised, nine-sixteenths were purple-flowered and the other 

 seven-sixteenths were white. This peculiar result can be ex- 

 plained if it is assumed that each strain had two pairs of genes 

 for flower color, that these pairs are in different chromosomes, 

 and that the gene products of the two dominants interact to 

 produce purple flowers, whereas all other gene combinations give 

 white flowers. If one gene pair is C and c and the other is P 

 and p, the original parents would have been ccPP and CCpp. 

 Both parents were homozygous as these strains had been raised 

 separately and inbred for a number of generations. When these 

 plants were crossed, the Fi was CcPp, and had purple flowers. 

 The F2 segregated as in Fig. 92. The F2 ratio is a regular di- 



