What is a Punnett square?

A Punnett square is a grid that predicts the genotypes of offspring from a cross between two parents. You write one parent's gametes along the top and the other parent's gametes down the side, then fill each cell by combining the row allele with the column allele. For a single gene the grid is 2×2, for two genes it is 4×4. Crossing Aa × Aa fills four cells with AA, Aa, Aa, aa, which is where the 1 : 2 : 1 genotype ratio comes from.

Why does Aa × Aa give a 1 : 2 : 1 genotype ratio?

Each Aa parent makes two kinds of gamete, A and a, in equal numbers. The 2×2 grid pairs them four ways: A×A gives AA, A×a and a×A both give Aa, and a×a gives aa. So out of four equally likely cells you get one AA, two Aa and one aa, which simplifies to 1 : 2 : 1. Collapsing genotypes into traits, three of the four show the dominant phenotype and one shows the recessive, giving the 3 : 1 ratio.

How do I read the phenotype ratio?

A phenotype is the visible trait, decided by dominance. If a genotype has at least one uppercase allele at a gene it shows the dominant trait, written A_ where the blank means either allele. Only the fully lowercase genotype aa shows the recessive trait. For Aa × Aa the genotypes AA, Aa, Aa are all A_ (dominant) and aa is recessive, so the phenotype ratio is 3 : 1. The tool labels every class this way so you can match it to a trait.

What does a dihybrid AaBb × AaBb cross produce?

Two genes assorting independently. Each AaBb parent makes four gametes, AB, Ab, aB and ab, so the grid is 4×4 with 16 cells. The phenotype ratio comes out 9 A_B_ : 3 A_bb : 3 aaB_ : 1 aabb, the famous 9 : 3 : 3 : 1. For example, crossing pea plants that are heterozygous for round-yellow seeds gives nine round-yellow, three round-green, three wrinkled-yellow and one wrinkled-green out of every sixteen offspring on average.

How does the tool work out the gametes?

It splits each genotype into its genes and takes one allele per gene. For one gene Aa the gametes are A and a. For two genes AaBb it takes every combination of one allele from each gene, giving AB, Ab, aB and ab. A homozygous gene like AA contributes only A, so AABb yields just AB and Ab. The gametes shown under the grid are exactly the row and column headers used to build it.

What is a test cross and how do I set one up?

A test cross pairs an individual showing the dominant trait with a fully recessive partner, like Aa × aa or AA × aa, to reveal whether the dominant parent is homozygous or heterozygous. Aa × aa gives half Aa and half aa, a 1 : 1 ratio with both genotypes and both phenotypes appearing. AA × aa gives all Aa, so every offspring shows the dominant trait. Seeing any recessive offspring at all tells you the dominant parent carried a hidden recessive allele.

Check biology homework before handing it in

You worked a monohybrid cross by hand and got 1 : 2 : 1, but the dihybrid problem feels shaky. Type both parents, read the genotype and phenotype ratios off the grid, and confirm your 9 : 3 : 3 : 1 before the deadline. Because the link carries both genotypes, you can paste the URL into a study-group chat and everyone opens the same worked square instead of retyping it.

Build a worked example for a lecture or worksheet

Teaching meiosis and Mendel next period and want a clean square on the board. Enter AaBb × AaBb, screenshot the 4×4 grid with the gametes labelled along the edges, and you have a figure that already shows the 16 cells collapsing into 9 : 3 : 3 : 1. Swap to Aa × aa to demonstrate a test cross in the same lesson without redrawing.

Predict coat colour or trait odds for a breeding plan

A hobby breeder pairing two carriers wants the odds a pup or kitten shows a recessive coat colour. Enter the carrier genotypes as Aa × Aa and the tool shows a 1 in 4 recessive phenotype, the same 25 percent a vet would quote. For two independent traits, run the dihybrid and read each combined outcome straight off the phenotype ratio.

Study for the AP Biology or MCAT genetics section

Genetics ratio questions are fast points if the square is automatic. Drill monohybrid, test cross and dihybrid setups, watch how the grid changes when a parent goes from heterozygous to homozygous, and lock in why a test cross to aa reveals a hidden allele. Each example button loads a canonical cross so you can rehearse the pattern, not the arithmetic.

Punnett Square Calculator — Genotype and Phenotype Ratios

Cross two genotypes — Aa × Aa or AaBb × AaBb — get the grid, the genotype ratio and the phenotype ratio, one-click copy, browser-only

Runs locally
Category Calculator
Best for Getting a realistic range before a purchase, plan, workout, or schedule decision.

Enter two parent genotypes — a single gene like Aa × Aa, or two genes like AaBb × AaBb. The tool builds the Punnett square, lists the offspring genotype ratio, and collapses it to a phenotype ratio (uppercase = dominant). A monohybrid cross gives a 2×2 grid, a dihybrid gives 4×4.

Parent 1 genotype

Parent 2 genotype

Punnett square

×	A	a
A	AA	Aa
a	Aa	aa

Parent 1 gametes: A, a · Parent 2 gametes: A, a

Genotype ratio

of 4

1/4 (25%)

2/4 (50%)

1/4 (25%)

genotype: 1 AA : 2 Aa : 1 aa (1 : 2 : 1)

Phenotype ratio

of 4

3/4 (75%)

1/4 (25%)

phenotype: 3 A_ : 1 aa (3 : 1)

Try an example

Every gamete, grid cell and ratio is computed in your browser with plain JavaScript. No genotype is uploaded or logged.

What this tool does

Free Punnett square calculator for monohybrid and dihybrid genetics crosses. Type two parent genotypes, a single gene like Aa × Aa or two genes like AaBb × AaBb, and the tool works out every gamete, fills the Punnett grid (2×2 for one gene, 4×4 for two), and reports both the offspring genotype ratio and the phenotype ratio. A monohybrid Aa × Aa gives the classic 1 AA : 2 Aa : 1 aa genotype split and a 3 : 1 dominant-to-recessive phenotype split; a dihybrid AaBb × AaBb gives the 9 : 3 : 3 : 1 phenotype ratio that shows up on every genetics exam. Uppercase letters are read as dominant and lowercase as recessive, so the phenotype labels follow standard Mendelian notation (A_, aabb and so on). Each cell, ratio bar and percentage is computed locally, you can copy the whole result with one click, and the shareable link reopens the exact same cross. Everything runs in your browser, nothing is uploaded.

Tool details

Input: Form fields; The page exposes text boxes, numeric controls, file pickers, or structured inputs depending on the tool.
Output: Live result + Copy; The result area focuses on usable output, with copy, download, or preview actions when supported.
Privacy: Browser-side processing; The main tool logic does not call an external API, so inputs normally stay in the current tab.
Save / share: Shareable URL state; Key settings are encoded in the URL so another person can reopen the same setup.
Performance budget: Initial JS <= 10 KB; No WASM budget is declared, keeping the tool quick to open on mobile.
Best fit: Calculator · Student; Category and role tags drive related tools, internal links, and quick fit checks.

How to use

1. Input

Paste or drop your content into the tool panel.
2. Process

Click the button. All processing is local in your browser.
3. Copy / Download

Copy the result or download to disk in one click.

How Punnett Square Calculator fits into your work

Use it for fast estimates, comparisons, and planning numbers before you make the final call.

Calculation jobs

Getting a realistic range before a purchase, plan, workout, or schedule decision.
Comparing scenarios by changing one input at a time.
Turning rough assumptions into a number you can discuss.

Calculation checks

Double-check units, dates, rates, and rounding assumptions.
Treat health, finance, tax, and legal outputs as planning aids, not professional advice.
Save the inputs that produced an important result so you can reproduce it later.

Good next steps

These links move the current task into a more complete workflow.

Real-world use cases

Check biology homework before handing it in
You worked a monohybrid cross by hand and got 1 : 2 : 1, but the dihybrid problem feels shaky. Type both parents, read the genotype and phenotype ratios off the grid, and confirm your 9 : 3 : 3 : 1 before the deadline. Because the link carries both genotypes, you can paste the URL into a study-group chat and everyone opens the same worked square instead of retyping it.
Build a worked example for a lecture or worksheet
Teaching meiosis and Mendel next period and want a clean square on the board. Enter AaBb × AaBb, screenshot the 4×4 grid with the gametes labelled along the edges, and you have a figure that already shows the 16 cells collapsing into 9 : 3 : 3 : 1. Swap to Aa × aa to demonstrate a test cross in the same lesson without redrawing.
Predict coat colour or trait odds for a breeding plan
A hobby breeder pairing two carriers wants the odds a pup or kitten shows a recessive coat colour. Enter the carrier genotypes as Aa × Aa and the tool shows a 1 in 4 recessive phenotype, the same 25 percent a vet would quote. For two independent traits, run the dihybrid and read each combined outcome straight off the phenotype ratio.
Study for the AP Biology or MCAT genetics section
Genetics ratio questions are fast points if the square is automatic. Drill monohybrid, test cross and dihybrid setups, watch how the grid changes when a parent goes from heterozygous to homozygous, and lock in why a test cross to aa reveals a hidden allele. Each example button loads a canonical cross so you can rehearse the pattern, not the arithmetic.

Common pitfalls

Confusing genotype ratio with phenotype ratio. Aa × Aa is 1:2:1 by genotype but 3:1 by phenotype, because the two Aa and one AA all look dominant. Always state which ratio a question is asking for before you answer.
Writing a gene with two different letters, like Ab, for one locus. A single gene uses one letter in upper and lower case, so a heterozygote is Aa, not Ab. Two genes are written AaBb. Mixing letters within a pair is the most common typo and the tool flags it.
Forgetting that a homozygous parent makes only one kind of gamete. AA contributes only A and aa only a, so a cross like AA × aa is not a 2×2 of four distinct cells, it collapses to a single offspring genotype Aa. The grid shrinks accordingly.

Privacy

Every step, the gametes, the grid cells, the genotype and phenotype ratios, is plain JavaScript running in your browser tab. No genotype you type is uploaded or logged anywhere. The one thing to know: the shareable link encodes both parent genotypes in the URL, so a link pasted into chat records those genotypes in the recipient server's access log. The notation is harmless, but if you would rather not, use the copy button and paste the text instead of sharing the URL.

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Punnett Square Calculator — Genotype and Phenotype Ratios

What this tool does

Tool details

How to use

1. Input

2. Process

3. Copy / Download

How Punnett Square Calculator fits into your work

Calculation jobs

Calculation checks

Good next steps

Real-world use cases

Check biology homework before handing it in

Build a worked example for a lecture or worksheet

Predict coat colour or trait odds for a breeding plan

Study for the AP Biology or MCAT genetics section

Common pitfalls

Privacy

FAQ

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