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Smooth and wrinkled peas (left) and a Punnet Square describing the outcome of the crossing of two heterozygous smooth pea plants (right).

Here's an example of the resulting offspring of a cross between a homozygous smooth and a homozygous wrinkled pea plants. Notice that dominant genes are represented by capitol letters while recessives are depicted with lower case letters. The Punnett Square, in the right of the figure, shows how you can determine the genetics of the offspring of a mating. In this case, the Punnett Square is showing the outcome of a cross between two heterozygous parent pea plants, each with a Ss genotype. Arrange the genes of each parent along the edge of the square then cross-multiply for each of the four conditions (e.g., S x S = SS, S x s = Ss, s x S = Ss, and s x s = ss). The number of genotypes produced by the cross multiplication will give you the proportion of genotypes and phenotypes that will result from a mating. In the above example, the mating of heterozygous parents produces, on average, a quarter of the offspring that are homozygous dominant (SS), a quarter of the offspring that are homozygous recessive (ss), and half of the offspring that are heterozygous (Ss). Phenotypically, though, this works out to three quarters of the offspring showing the dominant (e.g., smooth) trait and only one quarter the recessive (e.g., wrinkled) trait.

Pea Soup is a site that will take you through a brief biography of Mendel, explain some of his discoveries, and has a interactive pea breeding experiment you can play around with.