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.
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.
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.
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.
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Dominant %
75.0%
Recessive %
25.0%
Genotype Ratio
1:2:1
| 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 |