Dihybrid Genetic Cross


A dihybrid cross illustrates the potential results from crossing individuals that differ in regard to two traits. In this case, the Parents, or P generation, differ in both pea color and shape. A plant homozygous for wrinkled green peas is crossed with a plant homozygous for round yellow peas. Because both parents are homozygous for both traits, only gametes coding for wrinkled green peas gametes coding for round yellow peas are produced. A diagram of the cross between two members of the F1 generation called a “Punnett square,” is shown here. When the gametes from these pea plants are combined all of the offspring in the F1 generation are round and yellow. This is because the traits for round and yellow are dominant over the traits for green and wrinkled. Because all offspring within the F1 generation are heterozygous for both traits, both the males and females can produce four different gamete combinations: round yellow peas, wrinkled yellow peas, round green peas, and wrinkled green peas. A Punnett square is created to determine the F2 generation that results from this cross. Of the offspring produced within the F2 generation, 9 are round yellow peas, 3 are round green peas, 3 are wrinkled yellow peas, and 1 is a wrinkled green pea. In other words, a 9:3:3:1 ratio exists within the F2 generation. Mendel was able to use this observation to determine that factors assort independently of other pairs of factors. This has become known as the law of independent assortment.

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