Punnett Square Calculator
Create a Punnett square for a monohybrid cross between two parents. Enter the genotypes of each parent to see all possible offspring genotypes, phenotype ratios, and probabilities.
A Punnett square is the standard visual tool in introductory genetics for predicting offspring outcomes from a known parental cross. Invented by British geneticist Reginald Punnett in 1905, it works by arranging one parent's alleles across the top of a 2×2 grid and the other parent's alleles down the side, then filling in each cell with the combined genotype. The result shows every possible offspring combination and how likely each is.
This calculator builds the monohybrid (single-gene) Punnett square from your parental input. You choose each parent's two alleles (dominant A or recessive a), and the calculator shows the four cells, the resulting genotype ratio (e.g., 1 AA : 2 Aa : 1 aa for a heterozygous × heterozygous cross), and the phenotype ratio (3:1 for that classic case).
The Punnett square assumes the gene follows simple Mendelian inheritance: two alleles, one dominant, with each parent contributing one allele at random. Real genetics frequently violates these assumptions — incomplete dominance, codominance, sex-linkage, multiple alleles (like blood type), polygenic traits, and gene interactions all need more sophisticated tools.
Inputs
Results
Dominant %
75.0%
Recessive %
25.0%
Genotype Ratio
1:2:1
Punnett Square Results
| Parameter | Value |
|---|---|
| Parent 1 Genotype | Aa |
| Parent 2 Genotype | Aa |
| Cross | Aa x Aa |
| AA (Homozygous Dominant) | 1/4 = 25.0% |
| Aa (Heterozygous) | 2/4 = 50.0% |
| aa (Homozygous Recessive) | 1/4 = 25.0% |
| Dominant Phenotype | 75.0% |
| Recessive Phenotype | 25.0% |
| Genotype Ratio | 1 : 2 : 1 |
| Phenotype Ratio | 3 : 1 |
Formula
How to use this calculator
- Select the two alleles for Parent 1. "Dominant (A)" or "Recessive (a)" for each.
- Repeat for Parent 2.
- Read the resulting 2×2 grid and the genotype/phenotype ratios.
- For dihybrid (two-gene) crosses, you need a 4×4 grid with 16 cells — use the dihybrid Punnett square calculator instead (where two genes are independently inherited).
- For traits with incomplete dominance or codominance, the genotype ratio is correct but the phenotype interpretation changes (intermediate or both-expressed phenotypes).
Worked examples
Classic 3:1 ratio
Both parents heterozygous (Aa × Aa). Offspring genotypes: 1 AA : 2 Aa : 1 aa Phenotypes: 3 dominant : 1 recessive This is the ratio Mendel observed in his pea-plant experiments and is the foundation of modern genetics. Any single recessive trait passed by two carriers produces 25% affected offspring.
Carrier × affected
Heterozygous × recessive (Aa × aa). Offspring: 50% Aa, 50% aa Phenotypes: 50% dominant trait, 50% recessive trait This pattern shows up in genetic-counseling contexts for autosomal recessive disorders when one parent is a known carrier and the other is affected.
When to use this calculator
Use the Punnett square in introductory genetics, biology homework, or basic genetic-counseling reasoning. It's the right tool for: - Single-gene Mendelian traits - Predicting offspring ratios from known parental genotypes - Estimating carrier-status probabilities
It is NOT the right tool for: - Multi-gene traits (use multi-locus models) - Sex-linked inheritance (use sex-linked Punnett squares or pedigree analysis) - Continuous traits like height, weight, or skin color (polygenic, not Mendelian) - Complex traits influenced by environment (most common human conditions)
For real medical-genetics questions about disease risk, see a genetic counselor — Punnett squares give probabilities but don't account for incomplete penetrance, expressivity, or modifier genes.
Common mistakes to avoid
- Treating ratios as guarantees. A 3:1 ratio is the expected average across many crosses; any single litter or family can deviate substantially due to random sampling.
- Assuming dominant alleles are "more common" or "stronger." Dominance just means which allele's phenotype is expressed in heterozygotes — it has no relationship to allele frequency in a population.
- Confusing genotype ratio with phenotype ratio. For complete dominance the phenotype ratio collapses two genotypes into one (3:1 from 1:2:1).
- Ignoring incomplete dominance. Some traits show intermediate phenotype in heterozygotes (e.g., red × white snapdragon = pink), so 1:2:1 doesn't collapse.
- Skipping the X-linked case. For X-linked traits, sons inherit only from mom and the math differs from autosomal traits.
Frequently Asked Questions
Sources & further reading
- Mendelian inheritance — Genetics Home Reference — U.S. National Library of Medicine