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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.

Parent 1 genotype

Parent 2 genotype

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How it works

Each parent passes one of its two alleles at random · 4 equally likely combinations = allele₁ᴬ×allele₁ᴮ … · Dominant phenotype = any square containing an uppercase allele

A Punnett square crosses the two alleles of one parent against the two of the other, giving four equally likely offspring squares. Counting how the squares fall into genotypes gives the genotype ratio, while grouping every square that carries at least one dominant (uppercase) allele against the purely recessive ones gives the phenotype ratio. For a monohybrid cross of two heterozygotes this is the classic 1:2:1 genotype and 3:1 phenotype result.

Worked example

Crossing Aa × Aa, parent one contributes A or a and parent two contributes A or a, giving AA, Aa, aA and aa. Grouped, that is 1 AA : 2 Aa : 1 aa. Three of the four squares carry a dominant A, so the phenotype ratio is 3 dominant : 1 recessive — a 75% chance of the dominant trait.

Sources: Punnett square — Wikipedia

Frequently asked questions

How do I write a genotype?

Use two letters of the same gene, with a capital for the dominant allele and lowercase for the recessive one — for example AA (homozygous dominant), Aa (heterozygous) or aa (homozygous recessive).

Why is the phenotype ratio 3:1 for Aa × Aa?

Three of the four squares (AA, Aa, Aa) contain at least one dominant A allele and therefore show the dominant trait, while only aa shows the recessive trait — a 3:1 ratio, even though the genotype ratio is 1:2:1.

Does this handle two genes at once?

This tool models a single gene with two alleles (a monohybrid cross). A dihybrid cross of two genes uses a 4×4 square and produces the familiar 9:3:3:1 phenotype ratio, which requires tracking both genes together.

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Compute nPr and nCr — how many ways to arrange or choose r items out of n.

Educational estimate, not medical advice — verify formulas, units and assumptions for your specific case.