Dihybrid and polyhybrid crosses. Mendel’s third law
In a dihybrid cross the parents differ in two pairs of traits. Mendel crossed yellow smooth (AABB) and green wrinkled (aabb) peas: F1 is all AaBb (yellow smooth). An F1 plant produces four kinds of gametes, 25% each: AB, Ab, aB, ab; they combine in 4 × 4 = 16 ways (a 4×4 Punnett square). In F2 the phenotype ratio is 9 yellow smooth : 3 yellow wrinkled : 3 green smooth : 1 green wrinkled (9:3:3:1), and the genotypes fall into 9 classes (1:2:2:4:1:2:1:2:1); each trait taken separately splits 3:1. The third law (independent inheritance of traits): when organisms differing in two or more pairs of traits are crossed, each pair of traits is inherited independently of the others; this holds only when the allele pairs lie on different pairs of homologous chromosomes. In a polyhybrid, n pairs of heterozygous genes give 2ⁿ kinds of gametes (AaBb — 4, AaBbCc — 8). In the test cross AaBb × aabb four phenotypes appear in the ratio 1 : 1 : 1 : 1.
“The 4×4 grid”. Draw a 4×4 Punnett square in your notebook: AB, Ab, aB, ab on top and at the side; fill in the 16 squares, count the phenotype classes and check 9:3:3:1.