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Punnett Square Calculator

Build one-gene or two-gene Punnett squares from parent allele pairs and see the grid, genotype ratio, and phenotype ratio.

Cross type

Example: Aa

Example: Aa

Result

3:1

Aa x Aa gives a 3:1 phenotype ratio and a 1:2:1 genotype ratio

Quick Answer

A Punnett square calculator pairs each gamete from two parents. For Aa × Aa, each parent makes A and a gametes. The four cells are AA, Aa, Aa, and aa. The genotype ratio is 1:2:1. With complete dominance, three cells show the dominant trait. One shows the recessive trait, giving 3:1.

Punnett Square Results Explained: What The Grid Shows

A Punnett square grid shows how parent gene letters can pair in a cross, and the tool lists each parent's possible gametes. The grid joins each gamete from one parent with each choice from the other. A one-gene cross tracks one letter pair, while a two-gene cross tracks two pairs and assumes the genes sort on their own. Each cell shows one gene pair for a child, and the tool counts matching cells to make a reduced genotype ratio. Under complete dominance, one uppercase letter hides the matching lowercase letter, so the tool groups cells by visible trait and shows a phenotype ratio; students use both ratios to check class work and see why two gamete pairs may make the same gene pair.

The Punnett Square Method For Alleles, Gametes And Ratios

The method follows four rules to build every possible offspring genotype, then counts and reduces the results into ratios.

  • Each gamete receives one allele from each gene pair.
  • A monohybrid parent such as Aa makes gametes A and a.
  • A dihybrid parent such as AaBb makes AB, Ab, aB, and ab when the genes assort independently.
  • Each grid cell joins one gamete from each parent.

How To Use A Punnett Square Calculator In Four Steps

Inputs

  • Cross type: choose monohybrid for one gene or dihybrid for two genes.
  • Parent 1 genotype: enter paired alleles such as Aa or AaBb.
  • Parent 2 genotype: enter a genotype with the same gene letters.

Steps

  1. Pick the one-gene or two-gene cross type.
  2. Enter both parent genotypes with matching letter order.
  3. Check the gametes listed for each parent.
  4. Read the grid, genotype ratio, and trait ratio.

Monohybrid Punnett Square Example With All Cells Shown

Parent 1 = Aa and Parent 2 = Aa.

  1. List each parent's gametes: Parent 1 gametes are A and a; Parent 2 gametes are A and a.
  2. Combine every gamete pair: A × A = AA, A × a = Aa, a × A = Aa, a × a = aa.
  3. Count the genotypes: AA = 1 cell, Aa = 2 cells, aa = 1 cell. Genotype ratio: 1 AA : 2 Aa : 1 aa. The reduced ratio is 1:2:1.
  4. Group cells by phenotype: AA and both Aa cells show the dominant trait, while aa shows the recessive trait. Phenotype ratio: 3 dominant : 1 recessive. The reduced ratio is 3:1.

Genotype ratio 1:2:1 and phenotype ratio 3:1. No rounding is needed because the result uses whole-cell counts.

When A Punnett Square Gives A Useful Genetics Result

Use the tool for class crosses with one or two genes and paired alleles. The model needs complete dominance. The genes must sort on their own. The grid helps check gametes, allele-pair counts, and trait groups.

For other science tools, see the science calculators hub or look up genetics terms in the glossary.

Assumptions

  • Uppercase letters mark dominant alleles.
  • Lowercase letters mark recessive alleles.
  • Each gamete combination has equal chance in the model.
  • Dihybrid genes assort independently.

Limitations

  • The calculator handles complete dominance only.
  • The calculator does not model codominance or incomplete dominance.
  • Linked genes, crossing over, epistasis, and sex linkage need another method.
  • The grid predicts expected ratios, not the exact offspring in a small family.

In Practice

A common mistake is reading both ratios as the same result. AA and Aa are different gene pairs, but both show the dominant trait. Count the grid once by allele pair. Then group the same cells by visible trait.

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Frequently Asked Questions About Punnett Square Results

What Is The Difference Between Genotype And Phenotype?

Genotype names the allele pair a child may inherit, while phenotype names the trait that pair can show. Under complete dominance, AA and Aa are different pairs. Both show the dominant trait in this model.

Why Does Aa Appear Twice In An Aa By Aa Cross?

Aa appears twice because the A allele can come from either parent. The pairs A × a and a × A make the same gene pair. Keep both cells in the count, which gives two Aa results.

How Many Cells Does A Dihybrid Punnett Square Need?

An AaBb two-gene cross uses four gametes from each parent in this model. The grid has 4 × 4 = 16 cells. A parent with fewer gamete types makes a smaller grid, but each possible pair still needs a cell.

Does A Punnett Square Predict Exact Offspring Counts?

A Punnett square shows expected chances, not an exact set of births or seeds. Chance can make a small group differ from the grid ratio. A large group gives a better test of the expected pattern.

Can This Calculator Handle Linked Genes Or Codominance?

No, this tool is limited to complete dominance and genes that sort on their own. Linked genes can change gamete odds. Codominance and incomplete dominance change the trait groups, so those crosses need another model.

Sources

Last updated: . Reviewed for accuracy against the formula shown above.