XIX 



HEREDITY 



373 



self-fertilise, their offspring 

 in the F 3 generation were of 

 two kinds. One-third of 

 the Fo tails (pure dominants) 

 produced only tails ; two- 

 thirds of them (impure domi- 

 nants) produced tails and 

 dwarfs in the 3 : 1 propor- 

 tion. Thus the F 2 genera- 

 tion, resulting from the self- 

 fertilisation of the cross-bred 

 forms or hybrids (F x ), con- 

 sisted of 25 per cent, pure 

 dominants, 50 per cent, ex- 

 ternally similar impure 

 dominants, and 25 per cent, 

 pure recessives. This may 

 be expressed in the accom- 

 panying scheme. 



When the peculiar " waltz- 

 ing mice ' are crossed with 

 normal mice, the offspring 

 (Fj) are all normal. The 

 waltzing habit is recessive. 

 But when the offspring are 

 inbred, their progeny (F 2 ) 

 are normal mice and walt- 

 zers in the proportion of 

 3:1. The recessive walt- 

 zers of this generation might 

 be sold as pure waltzers, 

 for if paired with others like 

 themselves they will yield 

 only waltzers, and that in 



,/ 



spite of the fact that their 

 parents were normal, as also 

 was one of their grand- 

 parents ! But the normals 

 of the F a generations, though 



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