# What does my Punnett square show?

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.

- Page: https://www.acalculator.org/biology/punnett-square-calculator
- JSON spec: https://www.acalculator.org/biology/punnett-square-calculator.json
- Version: 7dac3039e292

## Default answer

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.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| genes | Genes | One gene (a monohybrid cross) or two genes (a dihybrid cross). |
| p1a | Parent 1, gene A | Parent 1’s two alleles of gene A; A is dominant over a. |
| p1b | Parent 1, gene B | Parent 1’s two alleles of gene B; B is dominant over b. |
| p2a | Parent 2, gene A | Parent 2’s two alleles of gene A. |
| p2b | Parent 2, gene B | Parent 2’s two alleles of gene B. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| genotypes | Genotype ratio | How often each offspring genotype appears in the square, in lowest terms. |
| phenotypes | Phenotype ratio | How often each trait combination appears; A_ means AA or Aa, which show the same dominant trait. |
| allDominant | Shows every dominant trait | The chance an offspring shows the dominant trait of every gene (A_, or A_B_). |
| firstOnly | Shows the dominant A trait only | For two genes: the chance of the dominant A trait with the recessive b trait (A_bb). |
| secondOnly | Shows the dominant B trait only | For two genes: the chance of the recessive a trait with the dominant B trait (aaB_). |
| allRecessive | Shows every recessive trait | The chance an offspring shows the recessive trait of every gene (aa, or aabb). |
| gametes | Gametes | The gametes each parent can pass on, each with equal chance. |
| square | Punnett square | Each row of the square: a gamete of parent 1 crossed with every gamete of parent 2. |

## 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.

## Assumptions

- 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.

## Worked examples

1. genes = 1, p1a = Aa, p2a = Aa gives genotypes = 1 AA : 2 Aa : 1 aa, phenotypes = 3 A_ : 1 aa, allDominant = 75%, allRecessive = 25%, 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 = 1, p1a = Aa, p2a = aa gives genotypes = 1 Aa : 1 aa, phenotypes = 1 A_ : 1 aa, allDominant = 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 = 1, p1a = AA, p2a = aa gives genotypes = 1 Aa, phenotypes = 1 A_, allDominant = 100%, allRecessive = 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 = 2, p1a = Aa, p1b = Bb, p2a = Aa, p2b = Bb gives phenotypes = 9 A_B_ : 3 A_bb : 3 aaB_ : 1 aabb, genotypes = 1 AABB : 2 AABb : 1 AAbb : 2 AaBB : 4 AaBb : 2 Aabb : 1 aaBB : 2 aaBb : 1 aabb, allDominant = 56.25%, firstOnly = 18.75%, secondOnly = 18.75%, allRecessive = 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 = 2, p1a = Aa, p1b = bb, p2a = aa, p2b = Bb gives phenotypes = 1 A_B_ : 1 A_bb : 1 aaB_ : 1 aabb, genotypes = 1 AaBb : 1 Aabb : 1 aaBb : 1 aabb.

## FAQ

### 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.

## Sources

- OpenStax, Biology 2e, section 12.2 Characteristics and Traits (Punnett square; 1 : 2 : 1 genotypes and 3 : 1 phenotypes in a monohybrid cross; the test cross), CC BY 4.0, retrieved 2026-10-02. https://openstax.org/books/biology-2e/pages/12-2-characteristics-and-traits
- OpenStax, Biology 2e, section 12.3 Laws of Inheritance (independent assortment; a 4 × 4 dihybrid Punnett square gives 9 : 3 : 3 : 1), CC BY 4.0, retrieved 2026-10-02. https://openstax.org/books/biology-2e/pages/12-3-laws-of-inheritance