VARIATION AND HEREDITY 13 



called the " recessive" character. But this was not all, 

 when the tall cross-bred peas were left to self-fertilize, 

 which corresponds to close inbreeding in animals, among 

 their progeny there were giants and dwarfs in the aver- 

 age proportions of 3 to 1. 



When the dwarfs of this F 2 generation were allowed 

 to self -fertilize, their offspring (F 3 ) were all dwarfs, and 

 further generations bred from them were also all dwarfs. 

 They are called pure recessives, being "pure" as regards 

 dwarfness. 



But when the giants of the F 2 generation were left to 

 self -fertilize, their offspring (F 3 ) were of two kinds: 

 one-third of them (pure dominants) produced giants 

 only; two-thirds of them (impure dominants) produced 

 giants in the proportion of 3 to 1. Thus the F 2 genera- 

 tion, resulting from the self-fertilization of the cross-bred 

 forms or hybrids (F x ), consisted of 25 per cent, pure 

 dominants, 50 per cent, impure dominants, and 25 per 

 cent, pure recessives. 10 



The law will be made clear by examining Figures 4, 

 5 and 6 in which the inheritance of the waltzing trait is 

 shown for mice, and the inheritance of colors is shown 

 for red and white four-o 'clocks. 



Figure 5 shows how the waltzing character is recessive 

 and absence of this character is dominant. In the first 

 generation a normal mouse (represented in black) is 

 crossed with a waltzing mouse (represented in white). 

 The result is all normal mice in the first filial (hybrid) 

 generation. When two mice of this generation are 

 crossed, they yield waltzing mice in the proportion of 

 one waltzing to three normal mice. When the waltzing 

 mice of this generation are mated they yield waltzing 



10 Thomson & Geddes, op. cit., p. 129. 



