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Ward's World+McGraw Hill Mendelian Genetics Activity

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Mendelism (continued) + ward ' s science 5100 West Henrietta Road • PO Box 92912 • Rochester, New York 14692-9012 • p: 800 962-2660 • wardsci.com Fig. 3: Schematic representation of a dihybrid cross. In this example, pea plants with contrasting seed color and seed shape are crossed. Specifically, plants yielding yellow-colored and round- shaped seeds are crossed with plants yielding green-colored and wrinkled-shaped seeds. The resultant 9:3:3:1 ratio of the offspring is known as the dihybrid ratio. (Copyright © McGraw Hill) Independent assortment Mendel extended his experiments to examine the inheritance of two characters simultaneously. Such a cross, involving two pairs of contrasting traits (such as seed color and seed shape), is known as a dihybrid cross (Fig. 3). For example, Mendel crossed pea plants yielding yellow-colored and round-shaped seeds with plants yielding green-colored and wrinkled-shaped seeds. All of the F1 offspring yielded yellow-colored and round-shaped seeds. When the F1 individuals were self-crossed, four types of offspring were pro- duced in the following proportions—9/16: yellow and round; 3/16: green and round; 3/16: yellow and wrinkled; 1/16: green and wrinkled. This 9:3:3:1 ratio is known as the dihybrid ratio. On the basis of similar results in other dihybrid crosses, Mendel proposed that, during gamete forma- tion, segregating pairs of factors assort independently of one another. As a result of segregation, each gamete receives one member of every pair of factors [this as- sumes that the factors (genes) are located on different chromosomes]. As a result of independent assortment, all possible combinations of gametes will be found in equal frequency. In other words, during gamete formation in the aforementioned example, round and wrinkled factors segregate into gametes independent- ly of whether they also contain yellow or green factors. Thus, the 9:3:3:1 ratio is the result of segregation, inde- pendent assortment, and random fertilization.

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