seed parent) and one r allele (from the wrinkled seed parent). Because the R allele is dominant to the r allele, the phenotype of all the seeds was round. The phenotypic ratio in this case of Monohybrid cross is 1. Second cross Mendel then allowed his hybrid peas to self-pollinate. The wrinkled trait—which did not appear in his hybrid generation—reappeared in 25% of the new crop of peas. Random union of equal numbers of R and r gametes produced an F2 generation with 25% RR and 50% Rr—both with the round phenotype—and 25% rr with the wrinkled phenotype. Third cross Mendel then allowed some of each phenotype in the F2 generation to self-pollinate. His results: All the wrinkled seeds in the F2 generation produced only wrinkled seeds in the F3. One-third (193/565) of the round F1 seeds produced only round seeds in the F3 generation, but two-thirds (372/565) of them produced both types of seeds in the F3 and—once again—in a 3:1 ratio. One-third of the round seeds and all of the wrinkled seeds in the F2 generation were homozygous and produced only seeds of the same phenotype. But two thirds of the round seeds in the F2 were heterozygous and their self-pollination produced both phenotypes in the ratio of a typical F1 cross. Phenotype ratios are approximate. The union of sperm and eggs is random. As the size of the sample gets larger, however, chance deviations become minimized and the ratios approach the theoretical predictions more closely. The table shows the actual seed production by ten of Mendel's F1 plants. While his individual plants deviated widely from the expected 3:1 ratio, the group as a whole approached it quite closely. Mendel's hypothesis To explain his results, Mendel formulated a hypothesis that included the following: In the organism, there is a pair of factors that controls the appearance of a given characteristic. (They are called genes.) The organism inherits these factors from its parents, one from each. A factor is transmitted from generation to generation as a discrete, unchanging unit. (The r factor in the F2 generation passed through the round-seeded F1 generation. In spite of this, the rr seeds in the F2 generation were no less wrinkled than those in the P generation.) When the gametes are formed, the factors separate and are distributed as units to each gamete. This statement is often called Mendel's rule of segregation. If