Dihybrid Crosses and Beyond Simple Dominance

Mendel's law of independent assortment states that genes for different traits are inherited independently of each other, as long as they sit on different chromosomes (or far apart on the same one). A dihybrid cross tracks two genes at once -- for example, crossing two pea plants heterozygous for both seed shape and seed color (RrYy x RrYy). Because the two genes assort independently, each parent produces four equally likely gamete types (RY, Ry, rY, ry), which combine in a 4x4 Punnett square of sixteen equally likely boxes. Working through that square gives the classic dihybrid phenotype ratio: 9 round-yellow to 3 round-green to 3 wrinkled-yellow to 1 wrinkled-green.

Not every trait follows Mendel's simple dominant/recessive pattern. In incomplete dominance, a heterozygote shows a blended, intermediate phenotype -- a red snapdragon crossed with a white one produces all pink offspring. In codominance, a heterozygote shows both parental phenotypes fully and simultaneously, with neither one blended away -- a roan cow with a mix of distinct red and white hairs, or a person with type AB blood, are both codominance. Multiple alleles means more than two versions of a gene exist across a population, even though any one individual still carries only two -- human ABO blood type has three alleles (IA, IB, and i) that combine into four possible blood types. Polygenic traits are controlled by many genes acting additively, producing continuous variation rather than a small number of discrete categories -- human height and skin color are commonly cited examples.