68 THE CONTROL OF LIFE 



and 25 per cent, pure recessives. Or, using the con- 

 tractions D and R for dominant and recessive respect- 

 ively, the Mendelian formula for the result of crossing 

 two pure-bred organisms differing in a pair of contrasted 

 characters is D x R = D (R). But if the D (R)'s be 

 mated, the result will be 25 per cent. D + 50 per cent. 

 D (R), + 25 per cent. R. 



One of the cases that Mendel began with was that of 

 two pure-bred races of peas which differed in stature — 

 a giant race and a dwarf race. When these were crossed 

 the offspring were not intermediates, like those of the 

 long-eared and short-eared rabbits already mentioned, 

 they were all tall. Thus Mendel spoke of tallness as 

 dominant and dwarfness as recessive in this case. 

 When the hybrid tall peas were left to self-fertilise 

 (as happens in peas, corresponding to inbreeding in 

 animals), the members of the second filial generation 

 (Fa) were with remarkable exactness in the following 

 proportions — 25 per cent, pure tails + 50 per cent, 

 impure tails, + 25 per cent, pure dwarfs. 



The two cases we have instanced illustrate what is 

 called complete dominance. That is to say, the offspring 

 of the normal mice and the waltzing mice were all 

 normal ; the offspring of the tall peas and the dwarf 

 peas were all tails. But it is not necessary that the 

 dominance in Mendelian inheritance should be complete. 

 Thus if a Black and a White Andalusian fowl be crossed, 

 the progeny are what are called Blue Andalusians, 

 illustrating incomplete dominance. These Blue Anda- 

 lusians might be hastily regarded as Blends, but the 



