Punnett square maker
Type the genotypes of two parents to see the Punnett square, the expected genotypes and phenotypes, and their ratios. Works for dominant and recessive alleles, codominance and sex-linked (X-linked) genes.
Free · no sign-up · works in your browser
| gametes | T | t |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
Genotypes: TT (1 in 4), Tt (2 in 4), tt (1 in 4) · ratio 1 : 2 : 1
Phenotypes: dominant trait: 75%, recessive trait: 25% · ratio 3 : 1
Each box has the same chance (1 in 4). Ratios are expected ratios: real families and small samples vary by chance.
How to write a genetic cross in the exam
- State the parents' phenotypes and genotypes.
- Show the gametes, each carrying one allele (circle them).
- Draw the Punnett square and give the offspring genotypes and phenotypes, with the ratio or probability.
Words to use correctly
- Homozygous: two identical alleles (TT or tt). Heterozygous: two different alleles (Tt).
- Dominant: shown in the phenotype when present. Recessive: only shown when there is no dominant allele.
- Codominance: both alleles show in the phenotype of a heterozygote, e.g. blood group AB.
Questions students ask
Why is a 3 : 1 ratio not always what you see?
Each offspring is a separate chance event. Ratios are what you expect on average; small numbers of offspring often differ.
How do I write sex-linked genotypes?
Put the allele on the X chromosome, e.g. XᴴXʰ for a carrier female and XʰY for an affected male. In this tool, type XHXh and XhY.