Punnett Square Calculator

What is a Punnett Square?

Invented by English geneticist Reginald C. Punnett in 1905, the Punnett Square is a square diagram that is used to predict the genotypes of a particular cross or breeding experiment. It visually displays all possible combinations of maternal alleles with paternal alleles to determine the statistical probabilities of offspring traits.

How the Punnett Square Works

Each parent's genotype is first split into haploid gametes: a monohybrid parent Aa produces two gamete types (A and a), while a dihybrid parent AaBb produces four (AB, Ab, aB, ab) through independent assortment. Every maternal gamete is then paired with every paternal gamete, and each pairing fills exactly one box of the grid with the offspring's diploid genotype. Counting identical boxes gives the genotypic ratio, and grouping those genotypes by visible trait — assuming an uppercase allele is dominant over a lowercase recessive allele — gives the phenotypic ratio.

Worked example (Aa × Aa): the gametes A and a from each parent fill the 2×2 grid with AA, Aa, Aa, and aa — a genotypic ratio of 1 AA : 2 Aa : 1 aa. Because AA and Aa both display the dominant trait, the offspring show a 3:1 dominant-to-recessive phenotypic ratio.

Mendel’s Laws of Inheritance

• Law of Segregation: During gamete formation, the two alleles for each gene segregate from each other so that each gamete carries only one allele for each gene.
• Law of Independent Assortment: Genes for different traits can segregate independently during the formation of gametes, forming the basis for the 9:3:3:1 ratio in dihybrid crosses.