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In addition to monohybrid crosses, Mendel also performed crosses in which two characteristics were examined simultaneously. Such a cross, involving two pairs of contrasting traits, is called a dihybrid cross. This tutorial shows how the results of these experiments led to Mendel's fourth postulate, called independent assortment.
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Mendel's Dihybrid Cross Revealed His Fourth Postulate: Independent Assortment (p. 44)
If pea plants having yellow seeds that are also round are bred with those having green seeds that are also wrinkled, what would happen?
Note that we now have two pairs of traits: yellow/green and round/wrinkled. Yellow is dominant to green and round is dominant to wrinkled. A cross involving two pairs of traits is called a dihybrid cross.
One can understand the results of a dihybrid cross if one considers it as consisting of two monohybrid crosses conducted separately. That is, the chance of any plant having yellow or green seeds is not influenced by the chance that this plant will also have round or wrinkled seeds.
From these considerations, Mendel deduced a fourth postulate: During gamete formation, segregating pairs of unit factors assort independently of each other.

Because the two pairs of contrasting traits are inherited independently, one can predict the frequencies of all possible F
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When two independent events occur simultaneously, the combined probability of the two outcomes is equal to the product of their individual probabilities of occurrence.
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A dihybrid cross allows two pairs of contrasting traits to be examined simultaneously. In
Mendel's experiments with dihybrid crosses, he observed that the two traits always segregated independently, leading him to propose a fourth postulate of genetic inheritance. The postulate of independent assortment states that during gamete formation, segregating pairs of unit factors assort independently of each other. Thus, the product law of probabilities can be applied to determine the frequencies of all possible genotypes and phenotypes of crosses involving two or more gene pairs through the forked-line method. This method considers each contrasting pair of traits separately and then combines the results.
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Define the law of independent assortment.
Use the forked-line method to predict the genotypes and phenotypes of crosses involving two or more gene pairs.
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dihybrid cross forked-line method independent assortment product law trihybrid cross