What is a dihybrid cross?

A dihybrid cross examines the inheritance of two separate genes simultaneously. Each gene has a dominant allele (uppercase) and a recessive allele (lowercase). By crossing two parents and applying Mendel's Law of Independent Assortment, you can predict the ratio of offspring phenotypes and genotypes.

What does the 9:3:3:1 ratio mean?

When both parents are heterozygous (AaBb x AaBb), the expected phenotype ratio is 9 offspring with both dominant traits : 3 with only the first dominant trait : 3 with only the second dominant trait : 1 with both recessive traits. This only holds when genes assort independently.

What genotype format should I enter?

Enter a 4-character genotype using two pairs of letters — e.g., AaBb, AABB, aabb, AAbb. The first two characters represent gene 1 (uppercase = dominant, lowercase = recessive) and the last two represent gene 2. Both alleles of each gene must be present.

Does this assume complete dominance?

Yes. The calculator uses complete dominance for both genes: one dominant allele (AA or Aa) produces the dominant phenotype, and only homozygous recessive (aa) produces the recessive phenotype.

What if the parents are not heterozygous?

The calculator handles any combination — AABB x aabb, AABb x aaBb, AaBb x aaBb, etc. The Punnett square and ratios update accordingly. Many non-AaBb x AaBb crosses will not produce the classic 9:3:3:1 ratio.

Can I use different gene letters?

Yes. You may use any two-letter pair — e.g., RrYy, TtGg. Just ensure uppercase and lowercase of the same letter are used for each gene (e.g., T/t and G/g), and the genotype is exactly 4 characters.

Dihybrid Ratio Calculator

Enter two parent genotypes to calculate the Punnett square, genotype frequencies, and 9:3:3:1 phenotype ratios for a dihybrid cross.

Parent Genotypes

Parent 1 Genotype

Parent 2 Genotype

Quick Presets

Phenotype Ratio

Genotype Frequencies

4 x 4 Punnett Square

Summary

A dihybrid cross involves two genes at independent loci, each with two alleles (dominant and recessive). When two heterozygous parents (AaBb x AaBb) are crossed, Mendel's Law of Independent Assortment predicts an offspring ratio of 9 dominant-dominant : 3 dominant-recessive : 3 recessive-dominant : 1 recessive-recessive. This calculator builds the full 4x4 Punnett square for any parent genotype pair and tabulates both genotype and phenotype frequencies.

How it works

Enter the genotype for Parent 1 (e.g. AaBb or AABB).
Enter the genotype for Parent 2 (e.g. AaBb or aabb).
The calculator determines all gametes each parent can produce via independent assortment.
A 4x4 Punnett square is built by combining every gamete from Parent 1 with every gamete from Parent 2.
Genotypes of all 16 offspring combinations are tallied and expressed as ratios.
Phenotypes are determined by dominance rules and summarized in the classic 9:3:3:1 (or simpler) ratio.

Use cases

Verify your manual Punnett square work for genetics homework.
Teach or learn Mendel's Law of Independent Assortment visually.
Quickly check offspring ratios for plant or animal breeding scenarios.
Explore how different parent genotypes (homozygous, heterozygous) change expected offspring ratios.
Generate phenotype probability percentages for any dihybrid combination.
Confirm that homozygous dominant x homozygous recessive produces all heterozygous offspring.

Frequently Asked Questions

Last updated: 2026-09-30 · Reviewed by Nham Vu