12 



CHAPTER 2 



FIGURE 2-1. Genotypic model pro- 

 posed to explain the phenotypic results 

 of certain crosses involving colored 

 and colorless flowered pea plants. 



CC X cc (Cross -fertilization) 



I I 



all C all c (Gametes) 



all Cc 



O^ 



were phenotypically uniform, as before. 

 Moreover, self-fertilization of the Fi gave 

 F2 which were proven to occur in the expected 

 1:2:1 genotypic ratio. 



Recall that the Cc phenotype is indis- 

 tinguishable from CC. In Cc individuals the 

 phenotypic expression of c is masked by the 

 expression of C. The ability of a gene to 

 express itself phenotypically in the presence 

 of a different allele is described in terms of 

 dominance. In the present case C is said to 

 be the dominant gene when present with c, 

 which is called, accordingly, recessive. It 



should also be noted particularly that the 

 concept of the gene so far developed has no 

 dependency upon the occurrence or non- 

 occurrence of dominance. Indeed, testing 

 our postulates has been made more compli- 

 cated by the fact that C is, for all intents and 

 purposes, completely dominant to c, since 

 our Fi Cc showed only the expression of C 

 and the presence of c was detected only upon 

 breeding Fi individuals. Also, only upon 

 further breeding of colored F2 were we able 

 to determine that )i. were CC and 73 Cc. 

 Dominance, then, refers to the phenotypic 



