GENES AND CHARACTERS 



319 



CR 



Cr 



cR 



cr 



CR 



Cr 



intermediate individuals are not all the same, but fall into graded 

 classes whose number depends ujjon the number of factors. The 

 F2 ratio is always a modification of some Mendelian ratio above the 

 monohybrid. 



Complementary Factors. Two factors that have no effect when 

 they occur separately l)ut produce a (U^finite character when 

 they are brought together in the same individual are said to be 

 complementary. 

 An example often 

 cited is that of 

 Bateson's white 

 sweet peas and 

 their red offspring. 



The two factors 

 in this case may 

 be interpreted as a 

 color factor C and 

 a red factor R. 

 The white parents cH 

 are C C r r and 

 ccRR, individuals 

 which produce 

 gametes bearing 

 the genes Cr and 



cR respectively, so jtk^, 184.— Diagram showing the behaviour of com- 



that the Fi hybrid plementary factors. The genotypic ratio is the same 



is heterozvffous for ^^ '" ^*^'^" ^''^ ^^ ^ normal dihybrid but the pheno- 

 , , , « , , , typic ratio is 9 : 7. 



both lactors. Its 



CcRr organization combines the two complementary factors and 



results in the production of red flowers. The F2 generation 



of this hybrid corresponds genotypically to a dihybrid, but since 



the presence of both factors is necessary for the production of 



color, all combinations of C and R with recessives of the opposite 



type included in the second and third groups of the 9:3:3:1 ratio 



are white, and a 9:7 ratio results (Fig. 184). 



Similar factors have been discovered in corn, rabbits and other 

 organisms. 



Supplementary Factors. This last group consists of those 

 which condition the expression of others without being essential 

 to the production of the character in question. In the sweet peas 



cr 



