But when two or more contrasting characters of the parents are followed to the F2 generation many permutations of these characters occur thus giving rise to a larger number of types of individuals than when a single pair of characters is concerned. When there is only one pair of contrasting characters there are usually but two types of offspring apparent in the F2 generation, viz., dominants and recessives in the ratio of 3 : 1 (Fig. 53); where there are two pairs of contrasting
Fig. 53. Monohybrid Diagram Showing Results of Crossing Round (R) Seeded with Wrinkled (W) Seeded Peas. Large circles represent zygotes, small ones, or single letters, gametes. In F1 all individuals are round but contain round and wrinkled gametes. In F2 the ♂ gametes are placed above the square, the ♀ ones to the left, and the possible combinations of ♂ and ♀ gametes are shown in the small squares, the relative numbers of different types being 1 RR : 2 R(W) : 1 WW.
Fig. 54. Dihybrid Diagram Showing Results of Crossing Peas Having Yellow-round (YR) Seeds with Others Having Green-wrinkled (GR) Ones. Four types of germ cells are formed by such a hybrid, viz., YR, YW, GR, GW, and the 16 possible combinations (genotypes) of these ♂ and ♀ gametes are shown in the small squares. Since recessive characters do not appear when mated with dominant ones these 16 genotypes produce 4 phenotypes in the following relative numbers: 9YR : 3YW : 3GR : 1GW. There is 1 pure dominant (upper left corner), 1 pure recessive (lower right corner), 4 homozygotes in diagonal line between these corners and 12 heterozygotes.
characters in the parents there are four types of offspring in the F2 generation in the ratio of (3 : 1)2 = 9 : 3 : 3 : 1; when there are three pairs of contrasting characters in the parents there are eight types of offspring apparent in the F2 generation in the proportions of (3 : 1)3 = 27 : 9 : 9 : 9 : 3 : 3 : 3 : 1, etc. Thus when Mendel crossed a variety of peas bearing round and yellow seeds with another variety having wrinkled and green seeds all the offspring of the F1 generation bore round and yellow seeds, round being dominant to wrinkled, and yellow to green.
