acalculator

What does my Punnett square show?

Choose one gene or two, then each parent’s alleles. The Punnett square calculator builds the square and gives the offspring genotype ratio, the phenotype ratio, and the chance of each trait.

Your numbers

Genes
Parent 1, gene A
Parent 2, gene A
Phenotype ratio
3 A_ : 1 aa

Crossing these parents gives phenotypes 3 A_ : 1 aa and genotypes 1 AA : 2 Aa : 1 aa.

Genotype ratio
1 AA : 2 Aa : 1 aa
Shows every dominant trait
75%
Shows every recessive trait
25%
Gametes
Parent 1: A, a; parent 2: A, a
Punnett square
A × (A, a): AA, Aa; a × (A, a): Aa, aa

Phenotype ratio: 3 A_ : 1 aa. Crossing these parents gives phenotypes 3 A_ : 1 aa and genotypes 1 AA : 2 Aa : 1 aa.

What will the offspring look like?

What is in each row of the Punnett square?

How to calculate

Builds the Punnett square for a monohybrid or dihybrid cross from the parents’ genotypes, with the offspring genotype and phenotype ratios and the chance of each phenotype.

Example with the default inputs (Genes One gene, Parent 1, gene A Aa, Parent 2, gene A Aa): Crossing these parents gives phenotypes 3 A_ : 1 aa and genotypes 1 AA : 2 Aa : 1 aa.

Method: Each parent passes one allele of each gene, every gamete with equal chance; the square crosses every gamete of parent 1 with every gamete of parent 2, and each cell is one equally likely offspring.

  • Complete dominance: AA and Aa show the same dominant trait; only aa shows the recessive trait.
  • With two genes, the genes assort independently (they are not linked on one chromosome).
  • Every gamete is equally likely, and every offspring equally likely to survive.

Machine-readable copies: Markdown, JSON.

Worked examples

Each example is checked against the calculator on every build.

  1. Genes One gene, Parent 1, gene A Aa, Parent 2, gene A Aa gives Genotype ratio 1 AA : 2 Aa : 1 aa, Phenotype ratio 3 A_ : 1 aa, Shows every dominant trait 75%, Shows every recessive trait 25%, Punnett square A × (A, a): AA, Aa; a × (A, a): Aa, aa.Source: OpenStax, Biology 2e, section 12.2 Characteristics and Traits (Punnett square; a monohybrid Yy × Yy cross gives 1 YY : 2 Yy : 1 yy and 3 yellow : 1 green; the test cross), https://openstax.org/books/biology-2e/pages/12-2-characteristics-and-traits, retrieved 2026-10-02
  2. Genes One gene, Parent 1, gene A Aa, Parent 2, gene A aa gives Genotype ratio 1 Aa : 1 aa, Phenotype ratio 1 A_ : 1 aa, Shows every dominant trait 50%.Source: OpenStax, Biology 2e, section 12.2 Characteristics and Traits (Punnett square; a monohybrid Yy × Yy cross gives 1 YY : 2 Yy : 1 yy and 3 yellow : 1 green; the test cross), https://openstax.org/books/biology-2e/pages/12-2-characteristics-and-traits, retrieved 2026-10-02
  3. Genes One gene, Parent 1, gene A AA, Parent 2, gene A aa gives Genotype ratio 1 Aa, Phenotype ratio 1 A_, Shows every dominant trait 100%, Shows every recessive trait 0%.Source: OpenStax, Biology 2e, section 12.2 Characteristics and Traits (Punnett square; a monohybrid Yy × Yy cross gives 1 YY : 2 Yy : 1 yy and 3 yellow : 1 green; the test cross), https://openstax.org/books/biology-2e/pages/12-2-characteristics-and-traits, retrieved 2026-10-02
  4. Genes Two genes, Parent 1, gene A Aa, Parent 1, gene B Bb, Parent 2, gene A Aa, Parent 2, gene B Bb gives Phenotype ratio 9 A_B_ : 3 A_bb : 3 aaB_ : 1 aabb, Genotype ratio 1 AABB : 2 AABb : 1 AAbb : 2 AaBB : 4 AaBb : 2 Aabb : 1 aaBB : 2 aaBb : 1 aabb, Shows every dominant trait 56.25%, Shows the dominant A trait only 18.75%, Shows the dominant B trait only 18.75%, Shows every recessive trait 6.25%, Gametes Parent 1: AB, Ab, aB, ab; parent 2: AB, Ab, aB, ab.Source: OpenStax, Biology 2e, section 12.3 Laws of Inheritance (a 4 × 4 Punnett square of a dihybrid cross gives 9 : 3 : 3 : 1), https://openstax.org/books/biology-2e/pages/12-3-laws-of-inheritance, retrieved 2026-10-02
  5. Genes Two genes, Parent 1, gene A Aa, Parent 1, gene B bb, Parent 2, gene A aa, Parent 2, gene B Bb gives Phenotype ratio 1 A_B_ : 1 A_bb : 1 aaB_ : 1 aabb, Genotype ratio 1 AaBb : 1 Aabb : 1 aaBb : 1 aabb.

How it works

Gene A has a dominant allele A and a recessive allele a; gene B has B and b. Each parent is AA, Aa or aa (and BB, Bb or bb for a two-gene cross).

  1. Gametes. Each parent passes one allele of each gene. With one gene, the gametes are the parent’s two alleles in the order typed (Aa gives A, a). With two genes, they are every pairing of an A allele with a B allele, in the order first A allele with first B allele, first A with second B, second A with first B, second A with second B (AaBb gives AB, Ab, aB, ab). Each gamete is equally likely.
  2. The square. Each gamete of parent 1 (rows) is crossed with each gamete of parent 2 (columns): 2 × 2 = 4 cells for one gene, 4 × 4 = 16 for two. Each cell is one equally likely offspring; in its genotype, each gene is written capital letter first (Aa, never aA).
  3. Phenotypes. With complete dominance, a gene shows its dominant trait when it has at least one capital letter (A_ is AA or Aa) and its recessive trait only as aa. With two genes, the genes assort independently.

The calculator shows:

  • Genotype ratio: the number of cells of each genotype, divided by their greatest common divisor, written n genotype and joined by :, ordered gene by gene with the capital-letter homozygote first (AA, Aa, aa; then AABB, AABb, AAbb, AaBB, …). Genotypes with no cells are left out.
  • Phenotype ratio: the same for the classes A_ and aa (one gene), or A_B_, A_bb, aaB_, aabb (two genes), in that order, leaving out empty classes.
  • Shows every dominant trait, Shows the dominant A trait only, Shows the dominant B trait only (two genes only), and Shows every recessive trait: each class’s cells ÷ all cells × 100%.
  • Gametes: Parent 1: …; parent 2: …, each list joined by , .
  • Punnett square: one row per gamete of parent 1, written gamete × (parent 2’s gametes): cells, rows joined by ; .

Worked examples by hand

Aa × Aa. Gametes A, a and A, a. Cells AA, Aa / Aa, aa: 1 AA : 2 Aa : 1 aa; phenotypes 3 A_ : 1 aa, so 75% show the dominant trait and 25% the recessive.

Aa × aa (test cross). Cells Aa, Aa / aa, aa: 1 Aa : 1 aa, 50% dominant.

AA × aa. Every cell is Aa: 1 Aa, 100% dominant.

AaBb × AaBb. Each parent makes AB, Ab, aB, ab, so the square has 16 cells: 1 AABB, 2 AABb, 1 AAbb, 2 AaBB, 4 AaBb, 2 Aabb, 1 aaBB, 2 aaBb, 1 aabb. Phenotypes 9 AB : 3 Abb : 3 aaB : 1 aabb: 56.25%, 18.75%, 18.75% and 6.25%.

Aabb × aaBb. Gametes Ab, ab and aB, ab: cells AaBb, Aabb / aaBb, aabb, so every phenotype is 1 in 4 (25%).

Other questions people ask

What is a Punnett square?

A grid that shows every way two parents’ gametes can combine. One parent’s gametes go along the side, the other’s along the top, and each cell is one equally likely offspring. Crossing Aa with Aa gives the cells AA, Aa, Aa and aa.

How do I read the genotype and phenotype ratios?

Count the cells. Aa × Aa gives 1 AA : 2 Aa : 1 aa. AA and Aa both show the dominant trait, so the phenotype ratio is 3 dominant : 1 recessive, a 75% chance of the dominant trait.

What does A_ mean?

An offspring with at least one dominant allele A: AA or Aa. Both show the same dominant trait, so A_ groups them. aa is the only genotype that shows the recessive trait.

What is a dihybrid cross?

A cross that follows two genes at once. Each parent with genotype AaBb makes four gametes (AB, Ab, aB, ab), so the square has 4 × 4 = 16 cells. The phenotypes come out 9 A_B_ : 3 A_bb : 3 aaB_ : 1 aabb.

What is a test cross?

Crossing an organism that shows the dominant trait with one that shows the recessive trait (aa). If the dominant parent is AA, every offspring is Aa; if it is Aa, half the offspring are aa. Pick Aa and aa to see the 1 : 1 result.

Does the calculator work for every trait?

It assumes complete dominance and, for two genes, independent assortment. Traits with incomplete dominance or codominance (such as ABO blood groups), linked genes, or many genes do not follow these ratios.