Punnett Square Calculator

How to use this calculator

1. Choose the two alleles carried by the first parent.
2. Choose the second parent’s genotype for the same locus.
3. Read genotype probabilities separately from the phenotype groups.

Formula and method

For the assumptions behind these results, read the explanation below. You can also explore Dihybrid Cross and Allele Frequency.

What a Punnett square represents

A parent with two copies of a locus transmits one allele to an offspring. In the simple Mendelian model used here, either allele of a heterozygote is equally likely to be transmitted. Combining the two possibilities from each parent gives four equally likely inheritance paths.

The paths are counted even when they produce identical genotypes. For example, Aa can result from A supplied by the first parent and a by the second, or from a supplied by the first and A by the second.

Reading genotype and phenotype

AA and aa are homozygous because both copies match. Aa is heterozygous because the copies differ. Capital and lowercase letters identify the two alleles; they are a naming convention and do not imply that one allele is more common or better.

The phenotype grouping assumes complete dominance: AA and Aa express the dominant trait, while aa expresses the recessive trait. Incomplete dominance, codominance, variable penetrance, and traits involving multiple genes need a more specific interpretation.

Probabilities across offspring

A probability describes the chance for an individual modeled offspring. It is not a schedule that forces each group of four offspring to contain the expected genotype counts. Repeated independent outcomes can produce streaks, especially in small samples.

This calculator models a single autosomal locus without differences in survival or transmission. Sex-linked inheritance, chromosome abnormalities, and other mechanisms can change the expected outcomes. For two independently assorting traits, the dihybrid calculator combines two single-locus crosses.