Dihybrid and polyhybrid crosses. Mendel’s third law
A cross between organisms that differ in two pairs of traits is a dihybrid cross. Mendel crossed yellow round (AABB) and green wrinkled (aabb) peas: all F1 were yellow round (AaBb). When F1 was crossed among themselves, F2 had four phenotypes in a 9 : 3 : 3 : 1 ratio: 9 yellow round, 3 yellow wrinkled, 3 green round, 1 green wrinkled. The third law (independent assortment of traits): each pair of traits is inherited independently of the others, because different allele pairs combine randomly in meiosis and give AB, Ab, aB and ab gametes with equal probability. F2 has 16 combinations (4 × 4), so a Punnett square is used. Taken separately, each trait still shows 3 : 1. The law holds only when the genes lie on different chromosomes. A cross for three or more pairs of traits is a polyhybrid cross: with n pairs F1 forms 2ⁿ kinds of gametes, and F2 has 2ⁿ phenotypic and 3ⁿ genotypic classes; for example, a trihybrid cross gives 8 kinds of gametes, 64 combinations and a 27 : 9 : 9 : 9 : 3 : 3 : 3 : 1 ratio.
“Gamete table”: groups write out the gametes formed by AaBb and AaBbCc organisms and compare their numbers.