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Punnett Square

General

Punnett Square

A grid diagram used to predict the probability of specific genotypes and phenotypes among offspring from a genetic cross between two parents.

Definition

A Punnett square is a simple grid diagram, developed by geneticist Reginald Punnett in 1905, used to predict the possible genotype combinations — and resulting phenotypes — that can appear among the offspring of a genetic cross between two parents. Each parent's possible gametes (the alleles they can pass on) are listed along the top and side of the grid, and each interior cell shows one possible offspring genotype formed by combining the corresponding row and column alleles.

For a monohybrid cross tracking a single gene with two alleles (commonly labeled dominant "A" and recessive "a"), the grid is 2x2 and produces four equally likely offspring combinations. Crossing two heterozygous parents (Aa × Aa) is the classic textbook example, producing a 1:2:1 genotype ratio (AA : Aa : aa) and a 3:1 phenotype ratio (dominant : recessive) under complete dominance. This is exactly what the Punnett Square Calculator computes automatically for any pair of parent genotypes.

For crosses involving two genes at once — a dihybrid cross — the grid expands to 4x4 with 16 possible combinations, following the principle of independent assortment (assuming the genes are on different chromosomes or far apart on the same one). The Dihybrid Cross Calculator handles this more complex case, producing the classic 9:3:3:1 phenotype ratio for two independently assorting traits.

Formula

A Punnett square doesn't use a single algebraic formula but rather combinatorial enumeration: for a cross involving n genes, each parent contributes 2ⁿ distinct gamete types (assuming heterozygosity at each locus), producing a grid of 2ⁿ × 2ⁿ cells and 4ⁿ total offspring combinations. For a monohybrid cross (n = 1), that's a 2×2 grid with 4 combinations; for a dihybrid cross (n = 2), it's a 4×4 grid with 16 combinations.

Worked Example

Consider a monohybrid cross between two heterozygous pea plants for seed color, where "Y" (yellow) is dominant over "y" (green): Yy × Yy.

The 2×2 grid produces: YY, Yy, Yy, yy — a genotype ratio of 1 YY : 2 Yy : 1 yy. Since Y is dominant, both YY and Yy plants appear yellow, giving a phenotype ratio of 3 yellow : 1 green, meaning each offspring has a 75% probability of yellow seeds and a 25% probability of green seeds.

Key Things to Know

  • Punnett squares assume independent, random fertilization: each gamete combination is treated as equally likely, which holds true for genes that follow simple Mendelian inheritance without linkage.
  • Dominant alleles mask recessive ones in the phenotype: a heterozygous genotype (Aa) displays the dominant trait, which is why genotype ratios (1:2:1) differ from phenotype ratios (3:1) under complete dominance.
  • Punnett squares connect directly to allele frequency: while a Punnett square predicts one cross, allele frequency describes how common each allele is across an entire population — the two concepts scale from individual crosses to population-wide genetics.
  • Larger grids handle more genes: a trihybrid cross (three genes) would require an 8×8 grid with 64 combinations, though in practice most calculators handle up to two genes for clarity.
  • Real inheritance can be more complex: traits with incomplete dominance, codominance, or genes linked on the same chromosome don't always follow the simple ratios a basic Punnett square predicts.

Related Calculators

Punnett Square Calculator →Build a 2x2 Punnett square for a monohybrid cross. Select each parent's genotype to get offspring genotype and phenotype ratios instantly, visually.
Dihybrid Cross Calculator →Build a 16-square dihybrid cross for two genes (AaBb x AaBb) and get the classic 9:3:3:1 phenotype ratio with a full genotype breakdown instantly.

Related Terms

Allele FrequencyAllele Frequency

Frequently Asked Questions

A Punnett square is used to predict the probability of specific genotype and phenotype combinations among the offspring of a genetic cross. It's a standard tool in introductory genetics for visualizing how alleles from two parents can combine.
You write one parent's alleles across the top of a grid and the other parent's alleles down the left side, then fill in each cell with the combination of the row and column allele. For a simple monohybrid cross, this produces a 2x2 grid; for a dihybrid cross involving two genes, it expands to a 4x4 grid.
The 3:1 ratio appears when two heterozygous parents (Aa × Aa) are crossed for a single trait with complete dominance. Of the four possible offspring combinations, three show the dominant phenotype (AA, Aa, Aa) and one shows the recessive phenotype (aa).
No — a Punnett square shows probabilities, not certainties, for any individual offspring. A 3:1 ratio means each offspring has a 75% chance of the dominant phenotype and a 25% chance of the recessive phenotype, but any single birth could go either way, similar to a coin flip not guaranteeing exactly 50/50 in a small sample.
A monohybrid cross tracks one gene with two alleles and uses a 2x2 grid with four possible combinations, while a dihybrid cross tracks two genes simultaneously and uses a 4x4 grid with 16 possible combinations. The dihybrid cross follows the same logic but requires tracking independent assortment between the two gene pairs.
A Punnett square models a single cross between two known parents, while Allele Frequency describes how common each allele is across an entire population. Population geneticists use allele frequencies to predict the probability of specific genotype combinations across many crosses, extending the same logic a Punnett square applies to one pair of parents.