Punnett Square Calculator

1. Enter Parent 1 genotype — type a genotype string such as AaBb or Aa. Each letter represents one allele; uppercase is dominant, lowercase is recessive. Every locus needs exactly two alleles (one pair of letters).
2. Enter Parent 2 genotype — use the same gene letters as Parent 1 in the same order. Both parents must have identical loci for the cross to be valid, though each can be homozygous (AA) or heterozygous (Aa).
3. Read the Punnett square — the grid updates instantly. Each cell shows the offspring genotype at the intersection of the two parent gametes. Scroll right for large dihybrid or trihybrid grids.
4. Check the distribution tables — genotype and phenotype counts, fractions, and percentages are listed below the grid. The reduced phenotype ratio (e.g. 9:3:3:1) appears at the bottom of the phenotype table.
5. Share or export — copy the shareable URL to send exact inputs to a colleague, or download a CSV of the distribution data for your records.

The genotype ratio tells you how often each exact combination of alleles appears among offspring. For a dihybrid cross of two AaBb parents, nine distinct genotypes are possible (AABB, AABb, AAbb, AaBB, AaBb, Aabb, aaBB, aaBb, aabb), and each appears a predictable number of times across the 16-cell grid.

The phenotype ratio groups genotypes by observable expression. Because dominance masks the recessive allele, AaBb and AABB both produce the same phenotype (A_ B_). The classic dihybrid result — 9:3:3:1 — means 9 offspring show both dominant traits, 3 show only trait A dominant, 3 show only trait B dominant, and 1 shows both recessive. Understanding this distinction is essential for interpreting colony screening results when you cannot always sequence every animal.

In practice, Mendelian ratios are theoretical expectations for large numbers of offspring. Small litters may deviate substantially from predicted ratios by chance. Tracking actual genotype outcomes across many litters — as Moustra does — lets you confirm your cross is behaving as expected and flag deviations that might indicate linkage or other effects.