By Rebecca E. – Updated Aug 30, 2022
A Punnett square is a diagram developed by English geneticist Reginald Punnett in the early 20th century to apply probability to the inheritance patterns first described by Gregor Mendel in the 19th century. While Mendel’s original experiments focused on pea plants, the principles are universally applicable to all complex organisms.
A Punnett square helps determine the statistical likelihood of each possible genotype (and therefore phenotype) in the offspring of two parents for one or more traits.
Most human traits are polygenic and non‑Mendelian, but several classic examples follow Mendel’s laws: albinism, Huntington’s disease, and blood type. In Mendel’s pea plants, each parent contributed two “factors” (now known as alleles) that were inherited by the offspring.
Mendel created pure‑line populations—plants that were homozygous for a single trait (e.g., YY for yellow seeds or yy for green seeds). These lines produced offspring with consistent, predictable traits, laying the groundwork for studying dominance and segregation.
His key discoveries were:
These laws explain why a cross between two homozygous plants (YY × yy) yields all heterozygous offspring (Yy) in the first generation, and why the second generation shows a 3:1 phenotypic ratio for a single Mendelian trait.
To calculate genotype probabilities for a single trait, draw a 2×2 grid. For two traits (a dihybrid cross), use a 4×4 grid; for three traits, an 8×8 grid. Each cell represents a possible combination of parental alleles.
While Punnett squares are powerful, they become less practical for traits influenced by multiple alleles, polygenic inheritance, or complex dominance relationships. In such cases, statistical models or genomic data may be required for accurate predictions.
For more detailed examples and practice problems, see our resource page.
