Dihybrid Crosses and Genetic Variation

Mendel did not stop at single genes. By following two characteristics at once he discovered how different genes are inherited relative to one another, revealing a second great law of heredity and providing a mechanism for the enormous variety seen in offspring.

The Dihybrid Cross

A dihybrid cross tracks two genes simultaneously. Mendel crossed pea plants differing in seed shape and seed color. Let $R$ (round) be dominant to $r$ (wrinkled), and $Y$ (yellow) dominant to $y$ (green). He began with true-breeding parents $RRYY$ (round yellow) and $rryy$ (wrinkled green). The $F_1$ generation were all $RrYy$, round and yellow, confirming both dominant alleles.

Law of Independent Assortment

Mendel's second law, the law of independent assortment, states that alleles of different genes segregate in