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Punnett Square Calculator

Predict offspring genotypes from two parents' alleles.

Result

1 AA : 2 Aa : 1 aa
75.00%
25.00%

How it is calculated

This is a monohybrid cross: one gene, two parents, each written as two letters (for example Aa), where a capital letter is the dominant allele and a lowercase letter the recessive one. Each parent passes on one of its two alleles with equal chance, so there are 2 × 2 = 4 equally likely combinations. The tool lists every combination, orders the letters in each genotype (capital first), counts identical genotypes and prints them as count genotype. The dominant percentage is the share of the four boxes containing at least one capital letter, dominant% = boxes with a capital / 4 × 100, and the recessive percentage is the rest. It assumes complete dominance and one gene on a normal chromosome; linked genes, incomplete dominance or sex-linked traits behave differently. Exactly two letters per parent are needed.

Worked example

Cross two heterozygous parents, Aa × Aa. The four boxes are AA, Aa, aA and aa. Since aA is the same as Aa, the counts are 1 AA, 2 Aa and 1 aa, printed as 1 AA : 2 Aa : 1 aa. Three of the four boxes (AA, Aa, Aa) carry a capital letter, so the dominant share is 3/4 = 75.00%, and the recessive share is 1/4 = 25.00%.

Try this example →

Common mistakes

  • Typing a letter and its case inconsistently (A and a are the same gene; A and B are different genes).
  • Entering more or fewer than two letters per parent, or mixing letters from two genes (a dihybrid cross needs a different method).
  • Reading the 3:1 phenotype outcome as a guarantee for four children; the percentages are probabilities for each offspring.
  • Confusing genotype ratio (1:2:1) with phenotype ratio (3:1).
  • Using it for traits that show incomplete dominance or are sex-linked.

About this calculator

This calculator builds a Punnett square for a monohybrid cross — combining the alleles of two parents at a single gene — to predict the genotype ratios (e.g. 1:2:1 for AA:Aa:aa from two heterozygous parents) and the resulting phenotype odds based on dominant/recessive inheritance.

Biology teachers and students use it to work through classic genetics problems, and hobby breeders of plants or animals use it to predict trait outcomes from a planned cross.

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Comments (1)

  • Editorial team

    Read the percentages per child, not per family: in an Aa × Aa cross every offspring has the same 25% chance of aa, whatever earlier children were. A quick sanity check is that the genotype counts add up to 4. The tool simply treats any capital letter as dominant, so entering Aa and Bb is not a dihybrid cross; keep both parents on one gene. Share a cross you are stuck on, and which trait it describes.

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