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Punnett Square Calculator

Build a Punnett square for one gene or two genes from two parents' genotypes. See the genotype and phenotype ratios of their offspring.

Level: Grades 7–8

  • Common Core standard 7.SP.C.8.bRepresent sample spaces for compound events using methods such as organized lists, tables and tree diagrams. For an event described in everyday language (e.g., "rolling double sixes"), identify the outcomes in the sample space which compose the event.
Printable worksheet

Punnett Square Calculator

Inputs
Cross
Parent 1, gene A
Parent 2, gene A

Result

Phenotype ratio

3 A_ : 1 aa

Aa × Aa gives a phenotype ratio of 3 A_ : 1 aa and a genotype ratio of 1 AA : 2 Aa : 1 aa.

Genotype ratio
1 AA : 2 Aa : 1 aa
Dominant trait (A_)
75%3 of 4 boxes
Recessive trait (aa)
25%1 of 4 boxes

See the step-by-step solution ↓

Punnett square for Aa × Aa: parent 1's gametes down the side, parent 2's across the top
Punnett square for Aa × Aa: parent 1's gametes down the side, parent 2's across the top
P1 ↓ / P2 →Aa
AAAAa
aAaaa

Step-by-step solution

  1. List each parent's gametes. Each parent passes on one of its two alleles, with equal chance

    Aa → A, a; Aa → A, a

  2. Pair every gamete of parent 1 with every gamete of parent 2. Each pair fills one box

    2×2=4

  3. Count each genotype in the 4 boxes, then shrink the ratio to its smallest whole numbers

    1 AA : 2 Aa : 1 aa

  4. Group boxes by visible trait. One dominant allele is enough to show the dominant trait, and _ stands for either allele

    3 A_ : 1 aa

  5. Chance of the A_ trait: its boxes out of all the boxes

    34=75%

  6. Chance of the aa trait: its boxes out of all the boxes

    14=25%

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

What this calculator does

See what traits two parents can pass on to their offspring, and how likely each one is. A Punnett square is a grid that shows every way the parents' gene versions can combine. Those versions of a gene are called alleles. Each parent's possible gametes (the egg or sperm cells, each carrying one allele per gene) run along one edge. Each box holds the genotype, or allele pair, formed when those two gametes meet. Every box is equally likely. So counting boxes gives the expected share of each genotype and of each visible trait.

This calculator handles a monohybrid cross, which follows one gene (A). It also handles a dihybrid cross, which follows two genes (A and B) at once. Choose each parent's genotype, and it draws the square and counts the outcomes. Then it reduces them to familiar ratios such as 3:1, 1:2:1 and 9:3:3:1.

Where it's used

Why this math matters outside the classroom:

  • Recessive traits. Two parents who each carry one recessive allele, Aa × Aa, have a 1 in 4 chance per child of an aa genotype.
  • Mendel's peas. Biology classes repeat Mendel's pea cross, where Tt × Tt gives tall and short plants in a 3:1 ratio.
  • Dihybrid crosses. A dihybrid cross of two AaBb parents gives the classic 9:3:3:1 phenotype ratio across 16 boxes.
  • Animal breeding. Dog and cattle breeders work out the coat color odds for a litter before choosing a pairing.
  • Testcrosses. Gardeners cross a plant of unknown genotype with a recessive one, and any recessive offspring reveal a hidden allele.

Grade level and standards: Grades 7–8; Common Core State Standards for Mathematics 7.SP.C.8.b. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.

How to use it

  1. Choose a monohybrid cross (one gene) or a dihybrid cross (two genes).
  2. Pick each parent's genotype for gene A, and for gene B in a dihybrid cross. Capital letters are dominant alleles, and small letters are recessive ones.
  3. Read the phenotype and genotype ratios, and the chance of each trait combination.
  4. Check the Punnett square table. Parent 1's gametes run down the side, and parent 2's run across the top.

What the inputs mean

Parent 1, gene A
AA is homozygous dominant (two dominant copies), Aa is heterozygous (one of each), and aa is homozygous recessive (two recessive copies). Swap in whatever letters your problem uses. Only the dominant and recessive pattern matters.
Parent 1, gene B
The second gene in a dihybrid cross. The calculator assumes it sits on a different chromosome from gene A. Or it sits far enough away that the two are passed on independently.

Formula and method

gametes per parent = 2^(number of genes)   (with repeats, e.g. AA → A, A)
boxes = gametes of parent 1 × gametes of parent 2   (4 for one gene, 16 for two)
P(outcome) = boxes with that outcome ÷ total boxes
Aa × Aa → 1 AA : 2 Aa : 1 aa, phenotypes 3 A_ : 1 aa
AaBb × AaBb → 9 A_B_ : 3 A_bb : 3 aaB_ : 1 aabb

P(outcome) means the probability, or chance, of that outcome. A and B stand for the dominant alleles of the two genes, and a and b for the recessive ones. Mendel's law of segregation says each parent passes on exactly one allele of each gene. Each of its two alleles is equally likely to be the one passed on. So a heterozygous Aa parent makes A and a gametes in equal numbers.

In a dihybrid cross, the law of independent assortment applies. It says the allele passed on for gene A has no effect on the one passed on for gene B. So each parent makes four gamete types in equal shares: AB, Ab, aB and ab for AaBb. That's why the square has 4 × 4 = 16 boxes.

Phenotypes, the visible traits, assume complete dominance. That means one dominant allele is enough to show the dominant trait. A_ means 'at least one A', with the blank standing for either allele. Ratios are reduced by the largest number that divides them all, so 8 : 8 is shown as 1 : 1.

Worked example

Two pea plants are both heterozygous for seed shape (Aa), where A (round) is dominant over a (wrinkled). What offspring do you expect?

  1. Each parent makes A and a gametes: Aa → A, a.
  2. The square has 2 × 2 = 4 boxes: AA, Aa, Aa and aa.
  3. Genotypes: 1 AA : 2 Aa : 1 aa.
  4. Three boxes have at least one A, so 3/4 = 75% are round and 1/4 = 25% are wrinkled.

Result: The phenotype ratio is 3 A_ : 1 aa (3 round : 1 wrinkled). The genotype ratio is 1 AA : 2 Aa : 1 aa.

Understanding the result

Ratios are chances for each offspring, not promises for a litter or a batch of seeds. A 3:1 ratio means each offspring has a 75% chance of the dominant trait. With only four offspring, getting 2 and 2, or 4 and 0, is quite possible.

The more offspring there are, the closer the real shares usually come to the predicted ratio. A chi-square test is a standard way to check a cross. It compares the counts you saw with the expected ratio to judge whether the cross behaved as predicted.

Assumptions and limitations

  • Complete dominance: heterozygotes (Aa) show the dominant trait.
  • Each gene has two alleles and follows simple Mendelian inheritance.
  • In a dihybrid cross, the two genes are passed on independently (they aren't linked).
  • All gametes work equally well, and every genotype survives equally well.
  • Some patterns produce different ratios and aren't modelled. These include incomplete dominance, codominance, multiple alleles, sex linkage, epistasis (one gene masking another) and linked genes.
  • Many real traits, including most in people and pets, depend on many genes and the environment. A single-gene square can't predict them.
  • Trihybrid (three-gene) and larger crosses aren't included.

Frequently asked questions

Why is the dihybrid ratio 9:3:3:1?

For each gene on its own, Aa × Aa gives 3/4 dominant and 1/4 recessive. When the genes are passed on independently, you multiply. That gives 3/4 × 3/4 = 9/16 dominant for both, and 3/4 × 1/4 = 3/16 for each mixed group. Last, 1/4 × 1/4 = 1/16 are recessive for both.

What is a test cross?

It's crossing an individual that shows the dominant trait with a homozygous recessive one (aa, or aabb for two genes). If any offspring show the recessive trait, the first parent must be heterozygous. Try Aa × aa here to see the 1:1 result, or AaBb × aabb for 1:1:1:1.

Why do the genotype and phenotype ratios differ?

AA and Aa look the same under complete dominance, so they fall into one phenotype group. The genotype ratio keeps them apart, and the phenotype ratio combines them.

Can I use other letters, like R and r?

Yes. The letters are only labels. Match your dominant allele to the capital letter and your recessive allele to the small one. Then read the results the same way.

How this calculator is checked

Method: Mendel's laws of segregation and independent assortment: enumerate equally likely gametes per parent, combine every pair, and count genotype and complete-dominance phenotype classes; ratios reduced by their greatest common divisor. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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