Punnett Square Calculator
Enter two parent genotypes to draw the Punnett square and get gametes, genotype ratios, and phenotype ratios for monohybrid, dihybrid, and trihybrid crosses.
Examples
| Parent 1 gametes down, parent 2 gametes across | AB | Ab | aB | ab |
|---|---|---|---|---|
| AB | ||||
| Ab | ||||
| aB | ||||
| ab |
- A dominant, B dominantA_ B_9/16 · 9/16 · 56.25%
- A dominant, B recessiveA_ bb3/16 · 3/16 · 18.75%
- A recessive, B dominantaa B_3/16 · 3/16 · 18.75%
- A recessive, B recessiveaa bb1/16 · 1/16 · 6.25%
What the calculator does
Everything a genetics problem set asks for, from the gametes to the final ratio
Each parent genotype is split into its allele combinations in textbook order (AB, Ab, aB, ab), so you can check the step before the grid.
One gene gives a 2 × 2 square, two genes a 4 × 4, and three genes an 8 × 8 with 64 offspring combinations, all counted automatically.
Counts are reduced to the classic ratios (1:2:1, 3:1, 9:3:3:1) and shown as fractions and percentages for probability questions.
Switch any gene to incomplete dominance or codominance and the heterozygote becomes its own phenotype, as in pink snapdragons or AB blood type.
Type the trait and phenotype names (Yellow / Green, Round / Wrinkled) and the phenotype table reads like the answer to the problem.
Click a genotype to see where it lands in the square, then copy the whole square and both ratio tables as plain text for notes or homework.
How a Punnett square works
A Punnett square turns Mendel's laws into arithmetic. During meiosis each parent passes on one allele of every gene, chosen at random, and under independent assortment the choice for one gene does not affect another. Listing every possible gamete of one parent along the top and every gamete of the other down the side, and filling each cell with the combination, enumerates every equally likely fertilisation. Counting cells therefore gives probabilities: with 16 cells, a genotype that appears in four of them has a 4/16 = 1/4 chance for each offspring.
For a single gene the square is 2 × 2. Crossing two heterozygotes, Aa × Aa, produces AA, Aa, Aa, and aa, the 1:2:1 genotype ratio. If A is completely dominant, AA and Aa look the same and the phenotype ratio collapses to 3:1. A test cross against a homozygous recessive (Aa × aa) gives 1:1 and is the classic way to reveal whether an individual showing the dominant trait is AA or Aa.
Two genes make a 4 × 4 square. The double heterozygote cross AaBb × AaBb yields the 9:3:3:1 phenotype ratio, which is simply the product of two 3:1 ratios: (3 + 1)(3 + 1) = 9 + 3 + 3 + 1. The same logic extends to three genes and an 8 × 8 square with a 27:9:9:9:3:3:3:1 ratio. The square is the exhaustive version of the multiplication rule; for larger problems geneticists usually multiply per-gene probabilities instead of drawing 256 cells, which is why this tool stops at three genes.
Dominance changes how genotypes map to phenotypes, not the genotypes themselves. Under incomplete dominance the heterozygote is intermediate, so red × white snapdragons give pink, and under codominance both alleles are expressed, as in the AB blood group. In either case the phenotype ratio equals the genotype ratio. This calculator lets you set the mode per gene, so a dihybrid cross can mix a completely dominant trait with an incompletely dominant one.
The ratios are expectations, not guarantees. A real litter of four from an Aa × Aa cross will often not contain exactly three dominant and one recessive offspring; the expected ratio emerges over many offspring, and a chi-square test is the usual way to judge whether observed counts fit it. Linked genes, sex-linked inheritance, epistasis, and lethal alleles all produce systematic departures from the simple square and need their own models.
Built and maintained by the runcell.dev team as part of our bioinformatics tool set. Last reviewed September 2026.
Frequently Asked Questions
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