Punnett Square Generator
The famous three-to-one ratio is not a rule to remember; it is what you get when you count the four squares of a heterozygous cross. This builds the grid from two parents, tallies the genotypes and the phenotypes directly from the cells, and confirms the shares sum to one — so the ratio is a result rather than a claim.
What this generator does
Pairs each allele from one parent with each from the other to fill the four cells, then tallies genotypes and phenotypes from those cells and expresses each as a share of four.
How to use this tool
- Choose each parent's genotype.
- Read the four cells of the grid.
- Check the ratios underneath, counted from those cells.
- Try Aa crossed with Aa for the classic result.
Understanding the controls
- The two parents
- Each is a pair of alleles: AA is homozygous dominant, Aa is heterozygous, aa is homozygous recessive. A capital letter is the dominant allele.
Common use cases
- Working through a monohybrid cross with the counting shown
- Teaching the difference between genotype and phenotype ratios
- Producing Punnett squares for a biology worksheet
- Checking a genetics answer against a computed one
- Seeing why two carriers can have an affected child
How this generator works
Each cell of the grid takes one allele from each parent, which is what a Punnett square represents: the equally likely combinations when each parent passes on one of its two alleles at random. Genotypes are tallied by counting identical cells, and phenotypes by whether a dominant allele is present at all. Before display every cell is checked to contain exactly one allele from each parent, the counts are recounted from the grid, and both sets of shares are confirmed to sum to one.
Randomness and fairness
Nothing here is random. Two parent genotypes give exactly one Punnett square.
For how randomness is produced across the whole site, see how Generate Random works.
Limitations and good to know
- A single gene with two alleles only — dihybrid crosses need a sixteen-cell grid and a different tool.
- Complete dominance is assumed, so incomplete dominance and codominance are not represented.
- The ratios are expected proportions, not what four actual offspring will be; small families deviate from them constantly.
- Sex linkage, linked genes and epistasis are all outside a single-gene square.
Privacy and your data
The cross is computed in your browser and your choices are not transmitted, stored or included in analytics.
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