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THE POPULAR SCIENCE MONTHLY 



These hybrids 3 when self -fertilized gave rise to a second filial genera- 

 tion of individuals some of which showed the dominant character and 

 others the recessive, the relative numbers of the two being approximately 

 three to one. Thus the hybrids produced by crossing yellow-seeded and 

 green-seeded peas yielded when self-fertilized 6,022 yellow seeds and 2,001 

 green seeds, or almost exactly three yellow to one green (Fig. 50). The 

 hybrids produced by crossing round and wrinkled seeded varieties 

 yielded in the second filial generation 5,474 round and 1,850 wrinkled 

 seeds, or approximately three round to one wrinkled (Fig. 53). The 

 hybrids from a tall and short stemmed cross produced in the second 

 filial generation 787 long stemmed and 277 short stemmed, or again ap- 

 proximately three tall to one short. And in every other case Mendel 

 found that the ratio of dominants to recessives in the second filial gen- 

 eration 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 homozygotes, or " extracted " dominants, and 

 when self-fertilized produce pure dominants indefinitely. On the other 

 hand, two thirds of the dominants are heterozygotes and when self-fer- 

 tilized give rise in the next generation to pure dominants, mixed domi- 

 nant-recessives and pure recessives in the proportion of 1:2:1. These 

 general results are summarized in the accompanying diagram (Fig. 51) 



Parent Generation 

 Fi a 



D E 



Homozygotes 

 Heterozygotes 



R 



Fig. 51. 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 

 mixed dominant-recessives (heterozygotes) are represented by circles half white and 

 half black. Successive generations are marked Fi F 2 , F 3 , etc. 



in which dominant characters are indicated by the letter D, recessive 

 characters by R, and mixed dominant-recessives, with the recessive char- 

 acter unexpressed, by D (R) ; while DD or RR indicate extracted domi- 

 nants or recessives, that is, pure dominants or recessives which have 



3 Bateson introduced the term homozygote for pure bred individuals re- 

 sulting from the union of gametes which are hereditarily similar, and heterozy- 

 gote for hybrids resulting from the union of hereditarily dissimilar gametes. 

 The gametes formed from a homozygote are all of the same hereditary type, 

 those formed from a heterozygote are of two or more different types. 



