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Dihybrid Cross Calculator

Cross two parents across two independent genes and get the phenotype ratio, the probability of each of the four trait combinations and the expected offspring counts for a chosen brood size.

Parent 1 genotype

Parent 2 genotype

Number of offspring

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How it works

Two genes assort independently · P(class) = P(gene-1 phenotype) × P(gene-2 phenotype) · Reduce the four probabilities ×16 to whole numbers for the ratio · Expected count = probability × brood size

A dihybrid cross follows two genes at once. Because the genes assort independently, you solve each gene as its own monohybrid cross and multiply the phenotype probabilities together — the product rule. Crossing two double-heterozygotes (AaBb × AaBb) gives each gene a 3:1 dominant-to-recessive split, and 3/4 × 1/4 combinations produce Mendel's classic 9:3:3:1 ratio. Scaling by the brood size turns the ratio into the counts you'd compare a real cross against.

Worked example

AaBb × AaBb: each gene gives 3/4 dominant and 1/4 recessive. So both-dominant = 3/4 × 3/4 = 9/16, each single-recessive class = 3/16, and both-recessive = 1/16 — the 9:3:3:1 ratio. Over 160 offspring that is roughly 90, 30, 30 and 10.

Sources: Dihybrid cross — Wikipedia

Frequently asked questions

Why is the classic dihybrid ratio 9:3:3:1?

Each of the two genes independently gives a 3:1 dominant-to-recessive phenotype split in a heterozygous cross. Multiplying those splits (3/4 and 1/4 in every combination) yields 9/16, 3/16, 3/16 and 1/16 — the 9:3:3:1 ratio.

Does this assume the genes are unlinked?

Yes. The product rule holds only when the two genes assort independently — on different chromosomes or far enough apart to recombine freely. Linked genes deviate from these ratios.

Can I cross parents that are not both AaBb?

Yes. Enter any four-letter genotype per parent, such as AABb or aaBb. The tool computes each gene’s dominant fraction separately, so it handles homozygous and heterozygous combinations alike.

Related calculators

Monohybrid Cross Calculator

Cross two parent genotypes for a single gene and get the genotype ratio, phenotype ratio, trait probabilities and the expected number of each offspring type in a chosen brood size.

Punnett Square Calculator

Cross two parent genotypes for a single gene and get the offspring genotype ratio, phenotype ratio and the odds of dominant versus recessive traits.

Test Cross Ratio Calculator

Cross an organism of unknown genotype against a fully recessive tester and get the expected phenotype ratio, number of phenotype classes and offspring counts that reveal how many genes are heterozygous.

Hardy-Weinberg Equilibrium Calculator

Work out allele and genotype frequencies under Hardy-Weinberg equilibrium from a dominant allele frequency p or from the frequency of the recessive phenotype q².

Educational estimate, not medical or laboratory advice — verify formulas, units and assumptions for your specific organism or protocol.