By Karen G Blaettler
Updated Aug 30, 2022
In 1905, Reginald Punnett released Mendelism, the first textbook of modern genetics. While studying inheritance, he devised a simple yet powerful visual tool—now known as the Punnett square—that predicts the probability of offspring genotypes and phenotypes.
Understanding a Punnett square starts with a few essential concepts.
A trait is any inherited characteristic, such as eye color or blood type. Each trait is governed by a gene, and an organism carries two copies of every gene—one from each parent. The different versions of a gene are called alleles. For instance, a person might inherit a blue‑eye allele from one parent and a brown‑eye allele from the other.
The genotype is the specific combination of alleles an individual possesses. The phenotype is the observable physical expression of that genotype. A person with one brown‑eye allele (B) and one blue‑eye allele (b) has a heterozygous genotype (Bb) and typically displays brown eyes, the dominant trait.
Alleles are classified as dominant or recessive. Dominant alleles mask the presence of recessive ones in heterozygous combinations. For example, the brown‑eye allele (B) dominates the blue‑eye allele (b). Some alleles, such as A and B in blood type, are co‑dominant, producing a combined phenotype (AB). Standard notation uses capital letters for dominant alleles and lowercase letters for recessive alleles.
Genetic interactions—linkage, incomplete dominance, epistasis—can cause phenotypes that deviate from simple predictions. A Punnett square offers a useful approximation but may not capture every biological nuance.
Follow these steps to construct and interpret a Punnett square for a single trait.
Before drawing the square, determine each parent’s genotype. If a parent’s genotype is unknown, information about grandparents can help infer it. For example, if a parent has brown eyes, at least one allele must be B, while the second allele could be B, G (green), or b (blue).
Generally, dominant traits appear more frequently in a population. However, local gene pools can shift these frequencies—for instance, blond hair may be more common in certain regions.
Sketch a square divided into four equal cells (a 2×2 tic‑tac‑toe grid). Optionally add a border for clarity.
Write one allele from Parent 1 along the top of the grid and the other allele from Parent 1 above the other column. Write one allele from Parent 2 along the left side and the other allele from Parent 2 beside it.
For each cell, combine the allele from the top with the allele from the side. The resulting pair represents a possible genotype of an offspring.
A cell containing two identical alleles (e.g., BB or bb) indicates a homozygous genotype. Different alleles (e.g., Bb) denote heterozygosity.
Assuming no other genetic factors, any cell that includes a dominant allele will express the dominant phenotype. In our eye‑color example, BB or Bb yield brown eyes, while bb results in blue eyes.
Count the occurrences of each genotype in the grid. For a cross between BB and Bb, the outcomes are BB, BB, Bb, and Bb—yielding a 50% chance of BB and 50% chance of Bb.
Count cells that carry the dominant allele. In the BB × Bb cross, all four cells contain B, so 100% of the offspring would display brown eyes.
Numerous web tools let you input parental genotypes and instantly generate the resulting genotype and phenotype tables, saving time and reducing errors.
