Punnett Square Calculator
Predict the genotype and simple dominant/recessive phenotype probabilities for a one-gene cross.
Parent genotypes
Predicted outcomes
Punnett square and genotype detail
| Father ↓ / Mother → | A | A |
|---|---|---|
| A | AA | AA |
| A | AA | AA |
How to use this Punnett square calculator
What this calculator does
This calculator models a single autosomal gene with two alleles: A, treated as dominant, and a, treated as recessive. It combines one allele from each parent in a 2 × 2 Punnett square, counts the four equally likely combinations, and reports the expected probabilities of the genotypes AA, Aa, and aa. It also groups AA and Aa into the dominant phenotype and aa into the recessive phenotype. The tool is appropriate for a simplified Mendelian teaching model; it does not determine an individual child's actual genotype, diagnose a condition, account for penetrance, or model traits influenced by many genes or environmental factors.
When to use it
Use the calculator to check homework on monohybrid crosses, compare carrier and non-carrier pairings, demonstrate why two heterozygous parents produce a 1:2:1 genotype ratio, or create a reproducible classroom worksheet in Excel. It is also useful for exploring a simplified autosomal recessive example, provided the parental genotypes are already known and the assumptions of complete dominance and equal allele transmission are reasonable.
How to calculate
- Select the Mother's genotype. Choose AA for two dominant alleles, Aa for one dominant and one recessive allele, or aa for two recessive alleles.
- Select the Father's genotype using the same notation. Results update immediately; no Calculate button is needed.
- Read the three genotype cards, then inspect the Punnett square to see the four allele pairings behind the percentages.
- Use the dominant and recessive phenotype summary pills to translate genotype probabilities into the simple complete-dominance phenotype model.
- Select Download Excel to export the current parent choices, outcome probabilities, and square. Select Reset to restore the default AA × AA cross.
Input guide
Mother's genotype is required and accepts one of three categorical values: AA, Aa, or aa. For example, choose Aa for a heterozygous mother. Switching from AA to Aa introduces a 50% chance that her transmitted allele is a; switching to aa makes that chance 100%. A common mistake is treating “Aa” and “aA” as different genotypes; this calculator uses the standard normalized form Aa.
Father's genotype is also required and uses the same three options. For example, choose Aa for a carrier father in a simplified recessive model. Changing this selection alters the father's two gamete labels and therefore two rows of the square. Do not infer a genotype solely from appearance when dominance, penetrance, or testing uncertainty is relevant.
Output guide
Chance for AA, Chance for Aa, and Chance for aa are exact probabilities within this four-cell model, displayed as percentages. AA is homozygous dominant, Aa is heterozygous, and aa is homozygous recessive. A zero means that genotype cannot arise from the selected parental alleles; 100% means every cell in the square has that genotype. The Most likely genotype reports the largest probability and shows ties when two outcomes are equally likely.
The Dominant phenotype summary equals AA plus Aa because either genotype contains at least one A allele. The Recessive phenotype summary equals aa. These are phenotype predictions only under complete dominance. The Punnett square lists the mother's two possible alleles across the columns, the father's across the rows, and the resulting offspring genotype in each cell. Each cell contributes 25 percentage points.
Worked example
Choose Aa for both parents. Each parent can transmit A with probability 1/2 and a with probability 1/2. Multiplying the independent transmission probabilities gives AA = 1/2 × 1/2 = 1/4, each Aa route = 1/4, and aa = 1/4. Combining the two heterozygous routes gives 25% AA, 50% Aa, and 25% aa. Under complete dominance, AA and Aa show the dominant phenotype, so the displayed phenotype probabilities are 75% dominant and 25% recessive.
Learn more
A Punnett square applies probability to possible gamete combinations; the LibreTexts explanation of the Punnett square approach describes that setup. For medical genetics context, review MedlinePlus Genetics inheritance patterns, which explains why real conditions may follow autosomal, X-linked, mitochondrial, or other patterns rather than this single simplified model.
How the probability model works
Each selected genotype contains two alleles. The calculator treats the two alleles as equally likely gametes, places one parent's possibilities on the columns and the other's on the rows, and forms four allele pairs. The canonical genotype order places the uppercase allele first, so a combination formed as aA is displayed as Aa. Counting occurrences gives the genotype probability:
Genotype probability = matching Punnett-square cells ÷ 4The model assumes segregation with equal transmission probability and complete dominance. The National Human Genome Research Institute's definition of dominance emphasizes that dominance describes a relationship between gene variants; it does not mean an allele is healthier, more common, or stronger in every biological sense.
For a recessive condition in a simplified two-allele model, a heterozygous individual may be called a carrier because one altered allele is present without the recessive phenotype. MedlinePlus explains the general concept of autosomal recessive inheritance and carriers. Clinical risk assessment can require molecular testing, pedigree analysis, ancestry-specific data, penetrance estimates, and professional genetic counseling.
Interpretation limits and common mistakes
Probabilities apply independently to each conception. A 25% outcome does not mean exactly one of every four children in a family must have that outcome, and previous births do not change the next modeled probability. The square also cannot infer unknown parental genotypes from phenotype alone when multiple genotypes can produce the same visible trait.
A one-gene Punnett square is not appropriate for height, most behavioral traits, many common diseases, or other polygenic outcomes. It also omits linkage, recombination, codominance, incomplete dominance, new variants, variable expressivity, and reduced penetrance. MedlinePlus provides a useful overview of how genetic risk is assessed in families and why a simple percentage may not capture every clinical factor.