86 



Heredity and Environment 



tion 787 long-stemmed and 277 short-stemmed plants, or again 

 approximately three 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. 



"Extracted" Dominants or Recessives. These recessives 

 derived from hybrid parents are pure and are known as "ex- 

 tracted" recessives; when self-fertilized they produce only re- 

 cessives. One-third of the dominants are also pure homozy- 

 gotes, or "extracted" dominants, and when self-fertilized pro- 

 duce only pure dominants. On the other hand two-thirds of 

 the dominants are heterozygotes and when self-fertilized give 

 rise in the next generation to pure dominants, dominant-reces- 

 sives and pure recessives in the proportion of 1:2:1. 



These general results are summarized in the accompanying 

 diagram (Fig. 27) in which dominant characters are indicated 

 by the letter D, recessive characters by R, and dominant-re- 

 cessives, with the recessive character unexpressed, by D (R) ; 

 while DD or RR indicate extracted dominants or recessives, that 

 is, pure dominants or recessives which have separated out from 

 dominant-recessives, D (R). The parental generation is usually 

 indicated by the letter P, and the successive filial generations by 

 F lf F 2 , F 3 , etc. 



Parent Generation 



r* 



Homozygotea 

 Helerozygotes 



Fa 



FIG. 27. DIAGRAM SHOWING RESULTS OF MENDELIAN SPLITTING where 

 the parents are pure dominants and pure recessives (homozygotes). All 

 pure dominants are represented by black circles, all pure recessives by 

 white ones, while dominant-recessives (heterozygotes) are represented 

 by circles half white and half black. Successive generations are marked 

 F,, F 2 , F lf etc. 



