Dihybrid Cross Punnett Square Calculator

Enter two parent genotypes for two genes (e.g. AaBb) to get the offspring genotype ratio, phenotype ratio, and full 4x4 Punnett square.

Use four letters: two alleles per gene, e.g. AaBb, AABb, or aabb. Capital = dominant allele, lowercase = recessive. Assumes the two genes assort independently.

Quick Facts

Method
Exact Mendelian gamete combination (4x4 Punnett square)
A heterozygous AaBb x AaBb cross gives the classic 9:3:3:1 phenotype ratio.

Your Results

Calculated
Phenotype ratio
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Dominant/recessive trait combinations
Genotype ratio
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Distinct genotypes among 16 offspring

Ready

Enter two four-letter genotypes and press Calculate.

About the Dihybrid Cross Punnett Square Calculator

A dihybrid cross tracks two genes at once. This calculator takes the genotypes of two parents — each written as four letters, two alleles per gene (for example AaBb) — and works out every possible offspring by combining their gametes in a 4×4 Punnett square. It returns the offspring genotype ratio, the phenotype ratio, and the full 16-cell grid so you can see exactly where each result comes from. The textbook case, a cross of two heterozygotes AaBb × AaBb, produces the famous 9:3:3:1 phenotype ratio.

How the dihybrid cross works

Each parent carries two alleles for gene 1 and two for gene 2. During meiosis those alleles separate, so a parent that is heterozygous for both genes (AaBb) can make four kinds of gametes in equal proportion: AB, Ab, aB, ab. Fertilization pairs one gamete from each parent, and 4 × 4 = 16 equally likely combinations fill the Punnett square. Counting the results by phenotype (when one allele is dominant) or by exact genotype gives the ratios below.

The 9:3:3:1 phenotype ratio

For AaBb × AaBb with complete dominance at both genes, the 16 offspring sort into four phenotype classes:

  • 9/16 show both dominant traits (A_B_)
  • 3/16 dominant for gene 1, recessive for gene 2 (A_bb)
  • 3/16 recessive for gene 1, dominant for gene 2 (aaB_)
  • 1/16 show both recessive traits (aabb)

The 9:3:3:1 ratio is really just two independent 3:1 monohybrid ratios multiplied together: (3:1) × (3:1). That is the product rule in action, and it only holds when the two genes assort independently.

The genotype ratio

Broken down by exact genotype rather than appearance, the same AaBb × AaBb cross gives a 1:2:1:2:4:2:1:2:1 ratio across nine genotypes: 1 AABB, 2 AABb, 1 AAbb, 2 AaBB, 4 AaBb, 2 Aabb, 1 aaBB, 2 aaBb, 1 aabb. The single most common offspring genotype is the double heterozygote AaBb, appearing in 4 of the 16 boxes.

Mendel's laws behind the numbers

  • Law of Segregation: the two alleles of a gene separate during gamete formation, so each gamete carries only one.
  • Law of Independent Assortment: alleles of different genes are distributed to gametes independently — the allele a gamete gets for gene 1 does not influence which it gets for gene 2. This is what makes all four gamete types equally likely and produces 9:3:3:1.

Frequently Asked Questions

What phenotype ratio does AaBb × AaBb give?
9:3:3:1. Out of every 16 offspring, on average 9 show both dominant traits, 3 show the first dominant and second recessive trait, 3 show the first recessive and second dominant trait, and 1 shows both recessive traits. It assumes complete dominance at both genes and independent assortment.
Why are there 16 boxes in a dihybrid Punnett square?
A parent heterozygous for two genes makes four kinds of gametes (AB, Ab, aB, ab). With four gamete types from each parent, there are 4 × 4 = 16 possible fertilization combinations, so the grid is 4 rows by 4 columns.
What if the parents are not both AaBb?
The calculator handles any genotypes. A parent homozygous for a gene (AA or aa) makes fewer distinct gametes, which changes the ratios — for example AaBb × aabb (a testcross) gives a 1:1:1:1 phenotype ratio instead of 9:3:3:1. Just enter each parent's actual four-letter genotype.
When does the 9:3:3:1 ratio break down?
It fails when the two genes are linked on the same chromosome (they do not assort independently), or when inheritance is not simple dominance — incomplete dominance, codominance, epistasis, sex-linkage, or lethal alleles all shift the observed ratios. This tool assumes two unlinked genes with complete dominance.

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