Punnett Square Calculator - Monohybrid Cross

Parent 1 Genotype

Parent 2 Genotype

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.

Why Use a Punnett Square Calculator?

Manually drawing a Punnett square and counting genotype outcomes is straightforward for one cross, but becomes tedious and error-prone when checking many combinations, for coursework, genetics problem sets, or breeding scenario planning.

This calculator instantly builds the grid and computes the ratio for any combination of parent genotypes, letting you explore how changing either parent's genotype shifts the offspring outcomes without redrawing the diagram each time.

The visual 2x2 grid, combined with the numeric genotype ratio and phenotype percentages, makes it easy to both verify homework answers and build intuition for how dominant and recessive alleles combine.

Who Should Use This Calculator?

Biology students learning Mendelian genetics and verifying Punnett square homework problems.

Teachers demonstrating genetic cross outcomes visually in the classroom.

Genetic counseling students building foundational intuition before tackling more complex pedigree analysis.

Animal and plant breeders predicting basic trait inheritance patterns for single-gene traits.

Science communicators and content creators needing quick, accurate genetics examples for educational material.

What Insights Does This Calculator Give You?

Genotype ratio, the primary result, showing the proportion of each genotype (AA, Aa, aa) expected among offspring, expressed as a simplified ratio.

Dominant vs. recessive phenotype split, the percentage of offspring expected to show the dominant versus recessive observable trait, which is often more relevant than genotype alone for predicting appearance or disease risk.

The full 2x2 grid, a visual breakdown of exactly which allele combinations from each parent produce each genotype, making the underlying probability logic transparent rather than just showing a final ratio.

How to use this Punnett Square calculator

Select Parent 1's genotype, choose AA, Aa, or aa from the dropdown.
Select Parent 2's genotype, choose AA, Aa, or aa from the dropdown.
Read the 2x2 grid, each cell shows the genotype resulting from combining one allele from each parent, color-coded by dominant versus recessive phenotype.
Check the genotype ratio and phenotype percentages, the result card shows the simplified genotype ratio alongside the dominant and recessive phenotype breakdown.

Show formula & methodology ↓Show less ↑

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

What is a Punnett square used for?

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.

What do AA, Aa, and aa mean?

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.

How does the calculator build the 2x2 grid?

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.

What is the genotype ratio for an Aa × Aa cross?

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.

What is a phenotype, and how is it different from genotype?

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.

What is the classic 3:1 phenotype ratio, and when does it occur?

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.

What happens with a homozygous recessive × homozygous recessive cross (aa × aa)?

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.

What happens with a homozygous dominant × homozygous recessive cross (AA × aa)?

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.

Can this calculator handle two genes at once (dihybrid crosses)?

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.

How does this relate to Hardy-Weinberg equilibrium?

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.

Are all four outcomes in a Punnett square equally likely?

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.

Why is this useful beyond basic biology class?

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