274 FOUNDATIONS OF BIOLOGY 



uals this proportion is merely approximate; the greater the 

 number of offspring, the closer it is approached. In this par- 

 ticular case Mendel obtained 787 dominant and 277 recessive 

 individuals. (Fig. 138.) 



Continuing the work, Mendel found that the dwarfs (reces- 

 sives) when inbred gave only recessives generation after 

 generation, and accordingly were 'pure', or EXTRACTED RECES- 

 SIVES. On the other hand, the tall plants (dominants) when 

 inbred proved to be of two kinds, one third pure EXTRACTED 

 DOMINANTS which bred true indefinitely, and two thirds 

 hybrids like their parents, giving when inbred the same ratio 

 of three dominants to one recessive in the THIRD FILIAL (F 3 ) 

 generation. 



Aside from his masterly foresight in realizing that success 

 depended on simplifying the problem by dealing with definite 

 contrasting characters, Mendel's claim to fame lies chiefly 

 in his discovery of a simple principle by which the results 

 may be explained. Since the hybrids when inbred always 

 give rise to hybrids and also to each of the parental types in a 

 pure form, it must be that the factors (genes) which deter- 

 mine the characters in question are SEGREGATED in the germ 

 cells. That is>, some germ cells bear one gene and other 

 germ cells the other, but one cell never bears both. If we 

 assume that the germ cells contain genes which determine 

 the size of the plant those of the original tall parent con- 

 taining the gene for tallness (S) , and those of the dwarf parent 

 the gene for dwarf ness (s) then the hybrids will arise from a 

 zygote which combines both genes (Ss), and since tallness is 

 dominant over dwarfness all will be tall Y Further, when 

 the germ cells of this hybrid (Ss) mature, if these genes 

 segregate so that, as a rule, half of the gametes bear S and 

 half bear s, then when such plants, each with this germinal 

 constitution, are inbred there will be equal chances for 



