What is a Punnett square?

A grid showing all possible offspring genotypes from a cross. The parents' gametes go along the top and side, and each cell is the combination.

What is the classic 3:1 ratio?

An Aa × Aa monohybrid cross gives 1 AA : 2 Aa : 1 aa (genotype ratio), which is 3 dominant : 1 recessive in phenotype, assuming complete dominance.

What is a test cross?

A cross between an organism of unknown genotype and a homozygous recessive (aa). The offspring phenotypes reveal whether the unknown parent was homozygous or heterozygous.

Does this calculator handle codominance?

Not directly. For codominance or incomplete dominance, the genotype ratio is still correct but the phenotype mapping is different (e.g. AB blood type, pink flowers). Read off the genotype ratio and apply the phenotype rules manually.

← HSC Biology calculators

NSW · HSCBiology

Monohybrid Punnett square calculator

Enter two parent genotypes for a single gene (e.g. Aa × Aa) and instantly see the 2×2 Punnett square, the genotype ratio, and the phenotype ratio.

Inputs

Parent 1 genotypeUppercase = dominant, lowercase = recessive. Example: AA, Aa, aa.Parent 2 genotype

Use the same letter for both alleles of the gene. Assumes complete dominance.

Result

Punnett square

	A	a
A	AA	Aa
a	Aa	aa

Genotype ratio

AA: 1/4 (25%)
Aa: 2/4 (50%)
aa: 1/4 (25%)

Phenotype ratio

Dominant: 3/4 (75%)
Recessive: 1/4 (25%)

How this calculator works

The calculator parses each parent into its two alleles, generates the unique gametes, fills in every combination in the grid, then counts the resulting genotypes and phenotypes assuming complete dominance (uppercase = dominant).

Common questions

What is a Punnett square?: A grid showing all possible offspring genotypes from a cross. The parents' gametes go along the top and side, and each cell is the combination.
What is the classic 3:1 ratio?: An Aa × Aa monohybrid cross gives 1 AA : 2 Aa : 1 aa (genotype ratio), which is 3 dominant : 1 recessive in phenotype, assuming complete dominance.
What is a test cross?: A cross between an organism of unknown genotype and a homozygous recessive (aa). The offspring phenotypes reveal whether the unknown parent was homozygous or heterozygous.
Does this calculator handle codominance?: Not directly. For codominance or incomplete dominance, the genotype ratio is still correct but the phenotype mapping is different (e.g. AB blood type, pink flowers). Read off the genotype ratio and apply the phenotype rules manually.