We have seen that segregation of different alleles of a single gene results in a 3:1 ratio of dominant:recessive phenotypes in the F₂ generation. What happens when the parental strains differ in two traits, for example when a strain having yellow and wrinkled seeds is crossed with a strain having green and round seeds? The results of these kinds of experiments constitute Mendel’s second key discovery, the principle of independent assortment. This principle states that segregation of one set of alleles of a gene pair is independent of the segregation of another set of alleles of a different gene pair. That is, each pair of alleles assorts (segregates) without affecting or being affected by the assortment of any other pair of alleles.