SCHEMATIC REPRESENTATION MENDEL'S LAW 343 



there are dominants which breed true to the dominant character, 

 and are therefore pure ; and thirdly, there are dominants which 

 may be called impure, and which on self-fertilisation (or in-breed- 

 ing, where the sexes are separate) give both dominant and re^ 

 cessive forms in the fixed proportion of three of the former to one 

 of the latter." 



Schematic Representation of Mendel's Law. Following Mr. 

 Punnett's suggestion, with slight modifications, we may use 

 the symbols P 1 , P 2 , P 3 for the parental, grandparental, and great- 

 grandparental generations ; F 1 for the first filial (hybrid) genera- 

 tions ; F 2 , F 3 , F 4 for the subsequent inbred generations. The 

 symbol D(R) means a dominant with the recessive character 

 unexpressed, but potentially present ; DD or RR means pure 

 " extracted " dominants or recessives i.e. those pure forms 

 which are sifted out from the inbreeding of " impure " dominants. 



D R . P 3 great-grandparental generation. 



D R . . P2 grandparental generation. 



D R . . P 1 parental generation. 



D(R) . . F 1 first filial (hybrid) generation. 



I DD 2 D(R) i RR . F 2 s e c o n d fi 1 i a 1 



" Extracted " pure Impure dominants. " Extracted " pure (inbred) generation, 

 dominants. recessives. 



i i i 



DD i DD 2D(R) i RR RR . F 3 third generation. 



DD DD i DD 2D(R) i RR RR RR . F 4 fourth generation. 



2. Theoretical Interpretation 



Mendel was not content with formulating his results in a 

 law ; he advanced a theoretical interpretation which is at once 



ingenious and simple, 



