dihybride

A dihybrid, in genetics, refers to an organism that is heterozygous for two different genes. It carries two different alleles for each gene, typically written as AaBb. Dihybrid analysis examines how two traits are inherited together and tests whether the genes assort independently during meiosis.

In a classic dihybrid cross, two individuals AaBb are crossed with another AaBb. When both genes show complete dominance and assort independently, the expected phenotypic ratio in the offspring's second generation (F2) is 9:3:3:1. This means nine display both dominant traits, three display the dominant form of the first trait and the recessive of the second, three display the recessive of the first trait and the dominant of the second, and one displays both recessive traits. The underlying mechanism is the formation of gametes in equal frequencies for the four allele combinations: AB, Ab, aB, and ab, illustrating Mendel’s law of independent assortment.

The dihybrid approach also highlights situations where the simple 9:3:3:1 ratio does not occur. If genes are linked on the same chromosome or interact epistatically, the offspring ratios deviate from the classic expectation. Dihybrid crosses thus serve as a foundational tool for studying genetic linkage, recombination, and gene interaction beyond a single trait.

A common classroom example uses seed shape and seed color in peas, with dominant alleles for shape (R) and color (Y) and recessive alleles (r, y). A dihybrid cross (AaBb × AaBb) is used to illustrate independent assortment and the 9:3:3:1 phenotypic outcome under ideal conditions.