374 THE STUDY OF ANIMAL LIFE CHAP. 



all alike in external appearance, turn out to be of two 

 kinds when they are used in breeding ; one-third of them 

 (pure dominants) will yield only normal mice ; the other 

 two-thirds (impure dominants) will split up again, when 

 inbred, into normal mice and waltzing mice in the old 

 average proportions of 3 : 1. 



Using D for organisms with the dominant character (or, 

 what will come to the same thing, with a character which 

 is absent in the organisms with which they are paired) ; 

 R for organisms with the corresponding recessive character 

 (or the absence of the character which the dominants 

 have) ; and D(R) for forms with dominant character 

 expressed and the recessive character latent (as sufrse- 

 quent breeding shows), the facts may be expressed, after 

 Punnett, in the following scheme :- 



Parents D R 



First Filial Generation (F x ) 



Second Filial Generation (F 2 ) ID + 2D(R) + 1R 



(pure (impure (pure 



dominants) dominants) recessives) 



I I ! 



Third Filial Generation (F 3 ) D ID +2D(R) + 1R R 



Mendelian inheritance is exhibited when the parent 

 forms have contrasted characters (allelomorphs) which 

 do not blend, or when one has a particular well-defined 

 character (say horns or a crest) which is absent in the 

 other. As we shall afterwards see, the important point 

 is not the difference in the expression of the character, 

 but the condition of the germ-cells as regards the factor, 

 determinant, or gene corresponding to the character. 

 Since the beginning of the twentieth century Mendelian 

 characters have been demonstrated in diverse types, of 

 which some illustration may be given. 



