Punnett Square Calculator — Mendelian Genetics Offspring Ratios
Enter two alleles from each parent and instantly see the 2×2 Punnett square, all possible offspring genotypes with their frequencies, and the dominant-to-recessive phenotype ratio.
| A | a | |
|---|---|---|
| A | AA | Aa |
| a | Aa | aa |
Offspring showing at least one dominant allele
How does this calculator work?
A 2×2 Punnett square crosses two parents' alleles to predict offspring genotype ratios. Each cell is equally probable (25%). Dominant phenotype shows when at least one uppercase allele is present; recessive only when both are lowercase. A classic Aa × Aa cross gives a 1 AA : 2 Aa : 1 aa genotype ratio and a 3:1 dominant-to-recessive phenotype ratio.
Formula
How this is calculated
A Punnett square models Mendel's law of segregation for a single gene with two alleles. Each parent passes one of their two alleles to each offspring at random with equal probability. The 2×2 grid maps every parent 1 allele (rows) against every parent 2 allele (columns), giving four equally likely offspring genotype combinations. If the same genotype appears more than once it has a proportionally higher probability.
This calculator uses the convention that an uppercase letter denotes a dominant allele and a lowercase letter denotes a recessive allele (same letter, different case — e.g. A and a). An offspring is dominant in phenotype if it carries at least one dominant (uppercase) allele; it shows the recessive phenotype only when both alleles are recessive. The classic Aa × Aa cross produces a 3:1 dominant-to-recessive phenotype ratio (AA : Aa : Aa : aa → 3 dominant : 1 recessive).
This tool models only simple (complete) dominance in a monohybrid cross. It does not cover codominance (e.g. blood type), incomplete dominance, sex-linked traits, dihybrid or polyhybrid crosses, epistasis or polygenic inheritance. Real genetic outcomes also depend on linkage and environmental factors not captured here.
Frequently asked questions
A Punnett square predicts the probability distribution of offspring genotypes and phenotypes when two parents with known alleles for a single gene reproduce. It is used in genetics education, animal breeding, genetic counselling and crop improvement.
A 3:1 phenotype ratio means 3 out of 4 offspring (75%) are expected to show the dominant trait and 1 out of 4 (25%) the recessive trait. It occurs when both parents are heterozygous (Aa × Aa) and dominance is complete.
Yes — just use the same letter in different cases to represent the two alleles of the same gene (e.g. B/b for brown/blue eyes, T/t for tall/short). The calculator assumes uppercase = dominant and lowercase = recessive. Using letters from different genes would be biologically meaningless but the math would still run.
Also known as
TG we-Calculate Editorial Team. (2026). Punnett Square Calculator — Mendelian Genetics Offspring Ratios [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/punnett-square-calculator
TG we-Calculate Editorial Team. "Punnett Square Calculator — Mendelian Genetics Offspring Ratios." TG we-Calculate. 2026. https://we-calculate.com/calculator/punnett-square-calculator.
TG we-Calculate Editorial Team, "Punnett Square Calculator — Mendelian Genetics Offspring Ratios," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/punnett-square-calculator
@misc{wecalculate_punnett_square_calculator, title = {Punnett Square Calculator — Mendelian Genetics Offspring Ratios}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/punnett-square-calculator}}, year = {2026}, note = {TG we-Calculate} }
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