Punnett Square Calculator

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Walk-through

How to Use This Calculator

5 steps

1

Choose monohybrid or dihybrid

Pick the Monohybrid tab for a single-gene cross (like Aa x Aa) or the Dihybrid tab for a two-gene cross (like AaBb x AaBb). Each tab has its own inputs and grid.

2

Enter both parent genotypes

Type each parent's genotype using one letter per gene: an uppercase letter for the dominant allele and the matching lowercase letter for the recessive allele — Aa, AA, or aa for monohybrid; AaBb for dihybrid (no spaces).

3

Read the grid

The Punnett square fills in instantly, combining every gamete each parent can produce into the full set of possible offspring genotypes — 4 cells for a monohybrid cross, 16 for a dihybrid cross.

4

Check the genotype and phenotype ratios

The result card reports both ratios: the exact genotype ratio (like 1:2:1) and the phenotype ratio that groups genotypes by which trait is actually visible (like 3:1 or 9:3:3:1).

5

Switch to the Ratios tab

See a bar chart of phenotype class counts and a full table listing every genotype, its phenotype class, count, and simplified ratio for your last computed cross.

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Reference

Formula & Methodology

3 formulas

Gamete combination

offspring genotype = gamete(parent 1) x gamete(parent 2)

Each parent's genotype splits into every possible gamete under independent assortment — one allele per gene. The Punnett square is the full cross-product of parent 1's gametes (rows) against parent 2's gametes (columns), one offspring genotype per cell.

Genotype and phenotype ratio

p^2 + 2pq + q^2 = 1 (monohybrid); 9:3:3:1 (dihybrid, two independent genes)

A monohybrid cross between two heterozygotes (Aa x Aa) gives a 1:2:1 genotype ratio (AA:Aa:aa), which collapses to a 3:1 phenotype ratio since AA and Aa both show the dominant trait. A dihybrid cross (AaBb x AaBb) combines two independent 3:1 splits into the classic 9:3:3:1 phenotype ratio.

Ratio simplification

ratio = count / gcd(all counts)

Every genotype and phenotype class count is divided by the greatest common divisor of all the counts in that tally, reducing the raw cell counts to the smallest whole-number ratio.

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Glossary

Key Terms Explained

8 terms

Allele One version of a gene. A gene for flower color might have a dominant allele (A, purple) and a recessive allele (a, white).

Genotype The actual pair of alleles an individual carries for a gene, like AA, Aa, or aa.

Phenotype The observable trait that results from a genotype. AA and Aa can have the same phenotype if A is dominant.

Dominant / recessive A dominant allele (uppercase) is expressed whenever it's present. A recessive allele (lowercase) is only expressed when paired with another recessive allele.

Monohybrid cross A genetic cross that tracks a single gene, like Aa x Aa. Produces a 2x2 Punnett square with 4 offspring combinations.

Dihybrid cross A genetic cross that tracks two independent genes at once, like AaBb x AaBb. Produces a 4x4 Punnett square with 16 offspring combinations.

Gamete A reproductive cell (egg or sperm/pollen) carrying exactly one allele per gene, formed when a parent's genotype splits during meiosis.

Homozygous / heterozygous Homozygous means both alleles for a gene match (AA or aa). Heterozygous means the two alleles differ (Aa).

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Scenarios

Real-World Examples

3 worked examples

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Classic monohybrid cross

Aa x Aa

Parent 1 Aa Parent 2 Aa

Two heterozygous parents produce a 1:2:1 genotype ratio (1 AA : 2 Aa : 1 aa). Since AA and Aa both show the dominant trait, the phenotype ratio is 3 dominant : 1 recessive — the textbook 3:1 ratio.

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Two-gene dihybrid cross

AaBb x AaBb

Parent 1 AaBb Parent 2 AaBb

Tracking two independent genes at once produces a 16-cell grid with 9 distinct genotype classes and a 9:3:3:1 phenotype ratio: 9 show both dominant traits, 3 show only the first, 3 show only the second, and 1 shows both recessive traits.

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Test cross

Aa x aa

Parent 1 Aa Parent 2 aa

Crossing a heterozygote with a homozygous recessive parent (a test cross) gives a 1:1 genotype ratio (Aa : aa), which is also a 1:1 phenotype ratio — half the offspring show the dominant trait, half show the recessive trait. This is exactly why test crosses reveal a parent's hidden genotype.

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Deep Dive

How Punnett Squares Predict Offspring Genetics

3 chapters · 4 min read

A Punnett square is the standard tool biology students and breeders use to predict the odds of a genetic cross before it happens, by laying out every combination of the parents' gametes in a grid. This calculator builds that grid automatically for monohybrid and dihybrid crosses and tallies the exact genotype and phenotype ratios so you can check your own hand-drawn square or explore crosses you haven't tried.

How the grid is built

Each parent's genotype splits into gametes during meiosis — a heterozygous Aa parent produces two types of gametes, A and a, in equal proportion. A Punnett square lines up one parent's possible gametes across the top and the other parent's down the side, then fills in every cell with the offspring genotype formed by combining that row's and column's gamete. A monohybrid cross (one gene) makes a 2x2 grid with 4 cells; a dihybrid cross (two genes, tracked with independent assortment) makes a 4x4 grid with 16 cells, since each parent now produces four gamete types instead of two.

Genotype ratio vs. phenotype ratio

The genotype ratio counts the exact allele combinations that appear in the grid — for Aa x Aa that's 1 AA : 2 Aa : 1 aa. The phenotype ratio groups those genotypes by which trait is actually visible: since a single dominant allele is enough to mask a recessive one, AA and Aa look identical, collapsing the 1:2:1 genotype ratio into a 3:1 phenotype ratio. This distinction is exactly why two individuals that look the same (both showing the dominant trait) can still have different genotypes underneath, and why a test cross — breeding with a known homozygous recessive (aa) — is the classic way to unmask which genotype an individual actually carries.

Why dihybrid crosses give 9:3:3:1

A dihybrid cross tracks two genes that assort independently, meaning the inheritance of one gene doesn't influence the other. Each gene on its own gives the familiar 3:1 dominant-to-recessive split. Combine two independent 3:1 splits and you get four phenotype classes in the proportions 3x3=9, 3x1=3, 1x3=3, and 1x1=1 — the classic 9:3:3:1 ratio that shows up across genetics coursework whenever two unlinked genes are crossed together.

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Questions

Frequently Asked Questions

7 questions

What is a Punnett square?+

A grid used to predict the genotypes and phenotypes of offspring from a genetic cross. Each parent's possible gametes label a row or column, and each cell shows the offspring genotype from combining that row's and column's gametes.

Why does Aa x Aa give a 3:1 ratio?+

A monohybrid cross between two heterozygous parents (Aa x Aa) produces a 1:2:1 genotype ratio (AA : Aa : aa). Since both AA and Aa show the dominant trait, the phenotype ratio collapses to 3 dominant : 1 recessive.

Why does a dihybrid cross give 9:3:3:1?+

A dihybrid cross (AaBb x AaBb) tracks two genes at once. Each gene independently gives a 3:1 dominant-to-recessive split, and combining two independent 3:1 splits produces the classic 9:3:3:1 phenotype ratio across the 16-cell grid.

What's the difference between genotype ratio and phenotype ratio?+

Genotype ratio counts the exact allele combinations (like AA, Aa, aa). Phenotype ratio groups those genotypes by which trait is actually visible — since a dominant allele masks a recessive one, AA and Aa look the same, collapsing a 1:2:1 genotype ratio into a 3:1 phenotype ratio.

What is a test cross?+

Crossing an individual of unknown genotype with a homozygous recessive parent (aa). If any offspring show the recessive trait, the unknown parent must carry a recessive allele (Aa); if all offspring show the dominant trait, the unknown parent is likely homozygous dominant (AA).

How should I type a genotype?+

Use one uppercase letter for the dominant allele and the matching lowercase letter for the recessive allele, with no spaces: Aa, AA, or aa for a monohybrid cross, AaBb for a dihybrid cross (two different genes, each written as its own letter pair).

Does this calculator handle sex-linked traits?+

Not in this version — it covers standard autosomal monohybrid and dihybrid crosses with simple complete dominance. Sex-linked and codominant/incomplete-dominance crosses follow different rules and aren't modeled here.

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Parent genotypes

Result

Parent genotypes

Result

Phenotype ratio

Genotype & phenotype classes

How to Use This Calculator

Choose monohybrid or dihybrid

Enter both parent genotypes

Read the grid

Check the genotype and phenotype ratios

Switch to the Ratios tab

Formula & Methodology

Key Terms Explained

Real-World Examples

Classic monohybrid cross

Two-gene dihybrid cross

Test cross

How Punnett Squares Predict Offspring Genetics

How the grid is built

Genotype ratio vs. phenotype ratio

Why dihybrid crosses give 9:3:3:1

Frequently Asked Questions

Get a branded PDF of your results

Punnett Square Calculator

Parent genotypes

Result

Parent genotypes

Result

Phenotype ratio

Genotype & phenotype classes

Related Calculators

How to Use This Calculator

Choose monohybrid or dihybrid

Enter both parent genotypes

Read the grid

Check the genotype and phenotype ratios

Switch to the Ratios tab

Formula & Methodology

Key Terms Explained

Real-World Examples

How Punnett Squares Predict Offspring Genetics

How the grid is built

Genotype ratio vs. phenotype ratio

Why dihybrid crosses give 9:3:3:1

Frequently Asked Questions

Get a branded PDF of your results

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