RULES AND RATIOS OF INHERITANCE 22^ 



tions 1:2: I. Here we have a simple example of what in 

 more complex cases is known as blending inheritance. 



B. Dependence on differences in several or many gene 

 pairs, having quantitative effects. "Blending Inheritance." 

 "Multiple Factors." 



As before seen, any characteristic is affected by many dif- 

 ferent genes, though if the parents differ in but a single one 

 of these gene pairs, the result is "single-gene" or "unit- 

 character" inheritance. But the parents may differ in two or 

 more of the genes affecting the characteristic, then yielding 

 inheritance in various complex ratios. The gene pairs in 

 which the parents differ may have qualitatively diverse ef- 

 fects, as in the case of eye color in Drosophila, or coat color 

 in rodents. 



But in other cases the different genes affecting the char- 

 acter differ only quantitatively in their effects. Thus, in 

 peas, one of the cases studied by Mendel was a cross between 

 tall and dwarf peas. These differed in a single gene pair; one 

 of these (AA) gave tall plants; the other (aa) gave short 

 plants. The gene A for tallness was dominant, so that the 

 heterozygotes Aa were tall. 



When such cases of quantitative difference in effects are 

 combined with the production of intermediate conditions in 

 the heterozygotes, and particularly when the parents differ 

 in several gene pairs, or "multiple factors," the result is to 

 produce many gradations of the characteristic, so giving 

 rise to what has been called blending inheritance. 



Such inheritance, as before remarked, is seen particularly 

 in such matters as dimensions, numerical characters, grada- 

 tions of color, and the like. But as just seen, in some cases 

 such characters depend on single-gene differences between 

 the parents; they then give typical Mendelian or sex-linked 

 ratios in the descendants. 



Dimensions thus giving typical single-gene inheritance 

 are: tallness and dwarfness in peas, in sweet peas, in An- 



