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Punnett Square Calculator

Biology

Build a 2x2 Punnett square for a monohybrid cross. Select each parent's genotype to get offspring genotype and phenotype ratios instantly, visually.

A
a
A
AA
Aa
a
Aa
aa
Dominant phenotype
Recessive phenotype

Genotype Ratio

1 AA : 2 Aa : 1 aa
Dominant Phenotype
75%
Recessive Phenotype
25%
Offspring Genotypes
AA, Aa, Aa, aa

What is a Punnett Square?

The Punnett Square Calculator builds a 2x2 grid showing the possible genotype combinations of offspring from a monohybrid genetic cross — a cross involving a single gene with two alleles. Select each parent's genotype (AA, Aa, or aa), and the calculator instantly displays the resulting grid, the genotype ratio, and the dominant versus recessive phenotype percentages.

This tool visualizes the classic technique developed from Gregor Mendel's pea plant experiments, still taught as the foundation of modern genetics. For population-level genetics involving allele frequencies across many individuals, see the Hardy-Weinberg Equilibrium Calculator.

How to use this Punnett Square calculator

  1. Select Parent 1's genotype — choose AA, Aa, or aa from the dropdown.

  2. Select Parent 2's genotype — choose AA, Aa, or aa from the dropdown.

  3. Read the 2x2 grid — each cell shows the genotype resulting from combining one allele from each parent, color-coded by dominant versus recessive phenotype.

  4. Check the genotype ratio and phenotype percentages — the result card shows the simplified genotype ratio alongside the dominant and recessive phenotype breakdown.

Formula & Methodology

Monohybrid cross logic:

1. Split each parent's genotype into its two individual alleles (e.g., "Aa" → A and a).
2. Combine each of Parent 1's alleles with each of Parent 2's alleles to fill the 2x2 grid (4 total combinations).
3. Count how many of the 4 cells produce each genotype (AA, Aa, aa) to get the genotype ratio.
4. Classify each genotype's phenotype: any genotype containing at least one dominant allele (AA or Aa) shows the dominant phenotype; only aa shows the recessive phenotype.

Worked example (Aa × Aa):

Parent 1 alleles: A, a. Parent 2 alleles: A, a.

Grid: AA, Aa, Aa, aa → genotype ratio 1 AA : 2 Aa : 1 aa

Phenotypes: 3 of 4 cells (AA, Aa, Aa) show the dominant phenotype → 75% dominant, 25% recessive

Note: This calculator models a single gene (monohybrid cross) with simple complete dominance. It doesn't account for incomplete dominance, codominance, sex-linked inheritance, or multiple interacting genes, which follow more complex inheritance patterns.

Frequently Asked Questions

A Punnett square is a diagram used in genetics to predict the possible genotype and phenotype combinations of offspring from a cross between two parents, based on the alleles each parent can pass on. This calculator builds the standard 2x2 grid for a monohybrid cross — a cross involving a single gene.
These represent genotypes for a single gene with two alleles: AA is homozygous dominant (two copies of the dominant allele), Aa is heterozygous (one dominant, one recessive allele), and aa is homozygous recessive (two copies of the recessive allele). By convention, the uppercase letter represents the dominant allele and the lowercase letter the recessive one.
Each parent's genotype is split into its two individual alleles, one contributed per gamete. The grid places one parent's alleles across the columns and the other's down the rows, and each of the four cells shows the genotype resulting from combining that row's allele with that column's allele — exactly as a real Punnett square works.
An Aa × Aa cross produces offspring in a 1 AA : 2 Aa : 1 aa ratio — meaning 25% homozygous dominant, 50% heterozygous, and 25% homozygous recessive, since each parent has a 50% chance of passing on either allele.
Genotype is the actual genetic makeup (e.g., Aa), while phenotype is the observable trait that genotype produces (e.g., "tall" versus "short"). Because the dominant allele masks the recessive one, both AA and Aa genotypes typically show the same dominant phenotype, while only aa shows the recessive phenotype — which is why this calculator reports dominant and recessive phenotype percentages separately from the genotype ratio.
The famous 3:1 phenotype ratio occurs specifically in an Aa × Aa cross, where 75% of offspring show the dominant phenotype (AA or Aa genotypes) and 25% show the recessive phenotype (aa genotype) — a foundational result from Gregor Mendel's original pea plant experiments.
An aa × aa cross produces 100% aa offspring, since both parents can only contribute a recessive allele — there's no dominant allele available anywhere in the cross, so all offspring show the recessive phenotype with certainty.
An AA × aa cross produces 100% Aa offspring — every offspring receives a dominant allele from one parent and a recessive allele from the other, resulting in a fully heterozygous first generation that all display the dominant phenotype, a classic setup in Mendelian genetics experiments.
This calculator focuses on the standard 2x2 monohybrid cross for a single gene, which covers the vast majority of introductory genetics problems. Dihybrid crosses (two genes, a 4x4 grid with 16 combinations) follow the same underlying logic but require tracking two independent allele pairs simultaneously.
A Punnett square predicts the outcome of one specific cross between two known parents, while the [Hardy-Weinberg Equilibrium Calculator](/hardy-weinberg-calculator/) predicts genotype frequencies across an entire population given allele frequencies, assuming no evolutionary pressures are acting. Both use the same p² + 2pq + q² logic, just applied at different scales.
Yes — a Punnett square assumes each parent's two alleles have an equal (50/50) chance of being passed on to any given offspring, so each of the four cells in the grid represents an equally likely (25% probability) outcome, though identical genotypes appearing in multiple cells increase that genotype's overall probability.
Beyond classroom genetics, Punnett square logic underlies practical applications like predicting inherited disease risk in genetic counseling, planning breeding programs in agriculture and animal husbandry, and understanding trait inheritance in plant breeding — all built on the same dominant/recessive allele framework demonstrated here.
Also known as
monohybrid cross calculatorgenetics square calculatorgenotype ratio calculatorphenotype ratio calculator