This Punnett square calculator predicts the offspring of a genetic cross. Enter each parent’s genotype — two letters for a monohybrid cross (Aa), four for a dihybrid cross (AaBb) — and get the full Punnett square with genotype and phenotype ratios.

What Is a Punnett Square?

A Punnett square is a grid that predicts the genetic outcomes of a cross between two parents. Each parent passes one allele (letter) per gene to the offspring; the grid lays out every combination of the parents’ gametes, so each cell is an equally likely offspring genotype. It was devised by Reginald Punnett in the early 1900s and remains the first genetics tool every biology student learns. This calculator builds the square for you — part of our biology collection, and a worked example of the probability rules in our Probability Calculator.

Monohybrid Cross Example (Verified)

Aa × Aa (two carriers of a recessive trait):

  • Genotypes: 1 AA : 2 Aa : 1 aa — 25% AA, 50% Aa, 25% aa
  • Phenotypes: 75% show the dominant trait (AA and Aa look the same), 25% show the recessive trait (aa)
  • This is the famous 3:1 phenotype ratio of Mendel’s pea experiments — and the reason two unaffected carrier parents can have an affected child with probability 1 in 4.

Dihybrid Cross Example (Verified)

AaBb × AaBb (two genes, assorting independently) produces the classic 9:3:3:1 phenotype ratio:

  • 9/16 dominant for both traits
  • 3/16 dominant A, recessive b
  • 3/16 recessive a, dominant B
  • 1/16 recessive for both (aabb)

Set both parents to AaBb above and the 16-cell square appears with every genotype counted.

How to Read the Square

  • Capital letter = dominant allele; lowercase = recessive allele.
  • Homozygous = two identical alleles (AA or aa); heterozygous = two different (Aa).
  • Each cell is equally likely, so probabilities are just cell counts ÷ total cells.
  • A recessive trait shows only in the homozygous-recessive genotype; carriers (Aa) look dominant but can pass the recessive allele on.

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Frequently Asked Questions

What is the difference between genotype and phenotype?

Genotype is the allele combination (Aa); phenotype is the observable trait it produces (e.g., purple flowers). Different genotypes (AA, Aa) can share one phenotype.

Why are offspring probabilities 25/50/25 for Aa × Aa?

Each parent passes A or a with 50% probability. Combining: AA = ½×½ = 25%, Aa = two orders × 25% = 50%, aa = 25%.

What cross produces a 1:1 ratio?

A test cross — heterozygous × homozygous recessive (Aa × aa) gives 50% Aa, 50% aa, revealing whether the dominant-looking parent is a carrier.

Do real traits always follow these ratios?

Simple Mendelian traits do, statistically, over many offspring. Incomplete dominance, codominance, linked genes and multiple genes per trait all bend the ratios — the square is the foundation those refinements build on.

Can this calculator do trihybrid crosses?

This version handles monohybrid (2 letters) and dihybrid (4 letters) crosses. Trihybrid squares have 64 cells and are best built gene by gene with the probability rules.

Is this Punnett square calculator free?

Yes — free, instant, no sign-up.