Punnett Square Calculator
Predicting Offspring Ratios From Two Genotypes
The Punnett square is the standard classroom and laboratory tool for tracking a single gene through a cross: line up one parent's alleles across the top, the other's down the side, and each cell shows one possible offspring genotype. Done by hand it's easy to mislabel a cell or miscount a ratio on anything beyond the simplest cross; this calculator builds the grid programmatically and tallies the exact genotype and phenotype ratios.
The Formula
Genotype Ratio = count of each genotype ÷ total offspring combinations
Phenotype = Dominant if any allele is uppercase, else Recessive
Every possible pairing of one allele from each parent is generated and normalized so the dominant (uppercase) allele is listed first — "Aa", never "aA".
Two Classic Crosses
| Cross | Genotype Ratio | Phenotype Ratio |
|---|---|---|
| Aa × Aa (heterozygous cross) | 1 AA : 2 Aa : 1 aa | 3 Dominant : 1 Recessive |
| Aa × aa (test cross) | 2 Aa : 2 aa | 1 Dominant : 1 Recessive |
These ratios describe the probability distribution across many offspring, not the guaranteed outcome of any single mating — a small number of actual offspring can deviate from the ratio by chance.
Where This Calculation Is Used
- Introductory genetics coursework — verifying manually worked Punnett squares and building intuition for dominant/recessive inheritance patterns.
- Plant and animal breeding programs — predicting the expected proportion of a desired trait in a cross before committing breeding resources.
- Test crosses — crossing an individual of unknown genotype against a homozygous recessive to determine whether it's heterozygous or homozygous dominant, based on the resulting ratio.
How to Use This Calculator
- Enter Parent 1's genotype as two letters, e.g.
Aa. - Enter Parent 2's genotype as two letters using the same gene letter, e.g.
aa. - Select Calculate to see the full Punnett square, genotype ratios, and phenotype ratios for the cross.
Related Calculations
Working with population-level allele data instead of a single cross? See the Allele Frequency Calculator or the Hardy-Weinberg Calculator.