Punnett Square Calculator
Cross two parents for a single gene and get the genotype and phenotype ratios of the offspring — a Punnett square worked out.
The odds of each genotype from a one-gene cross.
How the punnett square calculator works
Each parent passes on one of its two alleles at random. A Punnett square lists the four combinations; counting them gives the expected ratio. Aa × Aa gives 1 AA : 2 Aa : 1 aa — three dominant-phenotype offspring to one recessive. Pick the two genotypes and the calculator counts the square and reports the phenotype odds assuming complete dominance.
Formula: each combination of one allele from each parent has probability ¼
Worked examples
| Inputs | Dominant phenotype | Note |
|---|---|---|
| Aa × Aa | 75% | 1 : 2 : 1 — 75% dominant |
| Aa × aa (test cross) | 50% | 50 : 50 |
| AA × aa | 100% | all Aa — 100% dominant phenotype |
FAQFrequently asked questions
What is a test cross?
Crossing an individual of dominant phenotype with a homozygous recessive: if any offspring show the recessive trait, the parent was heterozygous.
Do real families follow the ratios?
Only on average — each child is an independent ¼ or ½ chance. Four children of Aa parents are not guaranteed one aa.
What about incomplete dominance or codominance?
The genotype ratios are the same; only the phenotypes differ (Aa shows a blend or both traits). Read the genotype rows.
Can it do two genes?
A dihybrid cross is a 16-cell square; for independent genes multiply the single-gene probabilities — run each gene here and multiply.
Where these figures come from
- IUPAC — Standard atomic weights (2021 conventional values) — the molar-mass table
- NIST — CODATA 2018 fundamental physical constants — Avogadro constant, gas constant, speed of light
- NIST Chemistry WebBook — thermochemical data
- National Institute of Standards and Technology — the US measurement and reference-data authority
Last checked: September 2026. Atomic masses are the IUPAC conventional values; constants are CODATA 2018; equations are the standard textbook forms.