Punnett Square

Definition

A Punnett Square is a diagram used in genetics to predict the possible genotypes and phenotypes of offspring from a cross between two parents. Developed by Reginald Punnett, it provides a simple way to organize and visualize how alleles combine during reproduction. By listing the potential alleles contributed by each parent and pairing them in a grid, geneticists can calculate the probabilities of different traits appearing in the offspring.

Example

Consider a cross between two heterozygous parents, each carrying one dominant and one recessive allele for a particular trait (Aa × Aa).

• Each parent contributes one allele—either A or a—to the offspring.

• When combined, the possible genetic outcomes are AA, Aa, Aa, and aa.

From these combinations:

• 25% of offspring (AA) will display the dominant trait.

• 50% (Aa) will also display the dominant trait but carry the recessive allele.

• 25% (aa) will express the recessive trait, which appears only when both alleles are recessive.

This example illustrates that even if both parents show the dominant trait, recessive traits can still appear in the next generation if both carry the recessive gene.

Why It Matters

The Punnett Square is a foundational tool in Mendelian genetics, offering a clear, systematic method for predicting hereditary outcomes. It helps explain how traits are passed from one generation to the next and why genetic variation occurs. This understanding has practical applications in agriculture (selective breeding for desired traits), medicine (genetic counseling and disease risk assessment), and evolutionary biology (tracking allele frequencies in populations). By making genetic probabilities visible and quantifiable, Punnett Squares bridge abstract genetic theory with observable biological outcomes.