In a dihybrid cross between two individuals heterozygous for two independently inherited genes (TtRr × TtRr), the expected phenotypic ratio is

Each gene independently gives a 3 : 1 ratio, and multiplying the two gives $(3 + 1)(3 + 1) = 9 : 3 : 3 : 1$ for the four phenotype combinations, illustrating Mendel's law of independent assortment. A 1 : 1 : 1 : 1 ratio arises from a dihybrid test cross with a double recessive.