230 HEREDITY AND ENVIRONMENT 



tall to one short. And in every other case 

 Mendel found that the ratio of dominants to 

 recessives in the second filial generation was 

 approximately three to one. These recessives 

 derived from hybrid parents are pure and are 

 known as "extracted" recessives; when self- 

 fertilized they produce recessives indefinitely. 

 One-third of the dominants are also pure hom- 

 ozygotes, or "extracted" dominants, and when 

 self-fertilized produce pure dominants indefi- 

 nitely. On the other hand two-thirds of the 

 dominants are heterozygotes and when self- 

 fertilized give rise in the next generation to 

 pure dominants, mixed dominant-recessives 

 and pure recessives in the proportion of 

 1 : 2 : 1. These general results are sum- 

 marized in the accompanying diagram (Fig. 

 49 ) in which dominant characters are indicated 

 by the letter D,, recessive characters by R, and 

 mixed dominant-recessives, with the recessive 

 character unexpressed, by D (R) ; while DD 

 or RR indicate extracted dominants or reces- 

 sives, that is, pure dominants or recessives 

 which have separated out from mixed domi- 

 nant-recessives, D (-K). The parental gener- 



