474 



BOTANY OF THE LIVING PLANT 



which cannot be distinguished from it by appearance, be represented 

 by (D) ; and if R represent the recessive, then the following will be 

 the scheme of inheritance : 



D X R 



D 



D D IDJ ID] R DID] JDJR R 



FIG. 395. 



Scheme of inheritance of Dominant, D, and Recessive, R, characters resulting 

 from the crossing of individuals which each bear one of a pair of differentiating 

 characters, through three generations / IF /% / 3 . (After Punnett.) 



In this scheme two pure strains, D, R, each possessing one of a pair of differen- 

 tiating characters, are crossed together. The resulting hybrids, Fi, all resemble 

 the dominant parent. When self-fertilised they give offspring 2, of which 

 one quarter bear the recessive, and three quarters the dominant character. 

 Of the latter, however, only one-third are pure dominants, giving when self- 

 fertilised, offspring in which the dominant character alone appears. The 

 remaining two-thirds are impure dominants, which on self-fertilisation 

 behave as the original Fi hybrids, yielding pure dominants, impure domin- 

 ants, and recessives in the proportions 1:2:1. This is true for all impure 

 dominants, no matter in which generation they occur. Both the " extracted " 

 pure dominants and the " extracted " pure recessives, which are formed in any 

 generation after a cross, breed true to the types of the original parents used in the 

 cross. 



In any sexually reproduced organism the gametes form the link 

 between successive adult generations. The characters peculiar to 

 the adult must therefore be represented in their constitution. In 

 the tall Pea some at least of the gametes, whether male or female, 

 must carry the tall character : for from an impure tall three quarters 



