What is a Punnett square?
A Punnett square is a visual diagram used in genetics to predict the genotypes and phenotypic ratios of offspring from a particular cross or breeding experiment. Named after Reginald C. Punnett, it's a simple grid that shows every possible combination of alleles from two parents.
How to use a Punnett square
For a monohybrid cross (single gene with two alleles), the Punnett square is a 2×2 grid. One parent's alleles are placed along the top, and the other parent's alleles are placed along the side. Each cell in the grid represents one possible offspring genotype.
For example, crossing two heterozygous parents (Aa × Aa) yields: 1 AA : 2 Aa : 1 aa, or a 3:1 dominant-to-recessive phenotypic ratio.
Understanding genotypic and phenotypic ratios
The genotypic ratio describes the proportion of each genotype (e.g., 1 AA : 2 Aa : 1 aa). The phenotypic ratio describes how the traits are expressed visually (e.g., 3 dominant : 1 recessive, assuming complete dominance).