Mendelism (continued)
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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.
