Punnett Square Calculator
Type two parent genotypes and this calculator crosses every allele combination to give the offspring genotypes and the phenotype ratio.
| Offspring genotypes | TT, Tt, Tt, tt |
|---|---|
| Dominant phenotype | 3 / 4 |
| Recessive phenotype | 1 / 4 |
Type each parent’s genotype as 2 letters, e.g. Tt × Tt. Capital = dominant allele, lowercase = recessive.
How to fill in a Punnett square by hand
- Write one parent's two alleles across the top of a 2×2 grid, and the other parent's two alleles down the side.
- Fill in each of the 4 boxes by combining the allele from its row and its column.
- Work out the phenotype of each box: if it contains the dominant (capital) allele, that trait shows; otherwise the recessive trait shows.
- Count how many of the 4 boxes give each phenotype to get the ratio.
Worked example: Tt × Tt
Each parent can pass on a T or a t. The 4 combinations are TT, Tt, Tt and tt. TT and Tt both show the dominant trait (because they contain a T), so 3 out of 4 offspring show the dominant trait and 1 out of 4shows the recessive trait — a 3 : 1 ratio.
Breed hundreds of real offspring and compare them against the predicted ratio with a chi-squared test in Genetics & Punnett Squares below.
Frequently asked questions
What is a Punnett square used for?
It predicts the possible genotypes (and their chances) of offspring from a genetic cross, by combining each parent’s two alleles in every possible pair.
What ratio do you get from crossing two heterozygous parents (Tt × Tt)?
A 3 : 1 ratio of dominant to recessive phenotype: 1 TT, 2 Tt and 1 tt, and TT/Tt both look the same (dominant), so 3 show the dominant trait and 1 shows the recessive trait.
What does it mean if an offspring is heterozygous?
It has two different alleles for that gene, one dominant and one recessive (like Tt). It shows the dominant trait but can still pass on the recessive allele.
Why do real results not always match the predicted ratio exactly?
Each offspring inherits its alleles independently, like a coin toss for each parent. With only a few offspring the ratio can look uneven; breeding many offspring brings it closer to the prediction.
