Page:Popular Science Monthly Volume 85.djvu/332

328 These hybrids when self-fertilized gave rise to a second filial generation 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 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. 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-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 summarized in the accompanying diagram (Fig. 51)

 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 F1 F2, F3, etc.

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 recessives, that is, pure dominants or recessives which have