What was the phenotypic ratio in Mendel's dihybrid cross?

The phenotypic ratio in Mendel’s classic dihybrid cross was 9:3:3:1. This is the expected distribution when two heterozygous individuals are crossed and the two genes assort independently, with complete dominance at both loci.

The four numbers represent phenotype categories rather than individual genotypes. Nine-sixteenths of the offspring are expected to show both dominant traits, three-sixteenths the first dominant and second recessive trait, three-sixteenths the first recessive and second dominant trait, and one-sixteenth both recessive traits.

Mendel’s example involved two pea characteristics: seed shape, round or wrinkled, and seed color, yellow or green. Treating each trait separately gives a 3:1 phenotypic ratio. Combining those probabilities produces the 9:3:3:1 pattern: 3:1 for one trait multiplied by 3:1 for the other.

The ratio is a prediction, not a guarantee for every cross. Small samples may deviate through chance, and real genetic systems can differ because of linkage, incomplete dominance, epistasis, or other non-Mendelian effects. The ratio is therefore most accurate under the ideal assumptions used in textbook genetics.