Predict genetic inheritance patterns using Punnett squares. Visualize monohybrid crosses and calculate genotype and phenotype ratios.
Last updated: March 2026
A Punnett square is a diagram used in genetics to predict the genotypes and phenotypes of offspring from a genetic cross. Named after British geneticist Reginald Punnett, who devised the approach in 1905, this tool remains fundamental in classical genetics education and breeding programs.
The square organizes the possible combinations of parental alleles in a grid format. Each parent contributes one allele for each gene, and the Punnett square shows all possible offspring combinations. Uppercase letters represent dominant alleles, while lowercase letters represent recessive alleles.
For a monohybrid cross (one gene with two alleles), a 2×2 Punnett square produces four possible offspring genotypes. The genotype is the genetic makeup (e.g., "Aa"), while the phenotype is the observable trait (e.g., "dominant" or "recessive" expression).
Classic Aa × Aa Cross (Heterozygous × Heterozygous)
Genotype refers to the genetic makeup (the actual alleles present, like 'Aa'), while phenotype refers to the observable trait or characteristic (like 'dominant' or 'recessive' expression). Multiple genotypes can produce the same phenotype.
Heterozygous means having two different alleles for a gene (e.g., 'Aa'). The individual carries both a dominant and a recessive allele. This typically results in the dominant phenotype being expressed.
Uppercase letters (A) represent dominant alleles that mask the effect of recessive alleles. Lowercase letters (a) represent recessive alleles that are only expressed when two copies are present (aa). This notation makes dominance relationships clear.
A 3:1 ratio is the classic phenotypic ratio from crossing two heterozygotes (Aa × Aa). Three offspring show the dominant phenotype (AA and Aa genotypes) and one shows the recessive phenotype (aa genotype).
Punnett squares show probabilities, not guarantees. They predict the likelihood of each outcome, but actual results may vary due to chance. With small sample sizes, observed ratios may differ from expected ratios.
A test cross involves breeding an individual with an unknown genotype (but dominant phenotype) with a homozygous recessive individual (aa). The offspring ratios reveal whether the unknown parent is AA or Aa.
Punnett squares work well for simple Mendelian traits controlled by a single gene with complete dominance. Complex traits involving multiple genes, incomplete dominance, codominance, or environmental factors require more sophisticated analysis.
Incomplete dominance occurs when the heterozygote (Aa) shows a blended phenotype between the two homozygotes. For example, red (AA) × white (aa) flowers producing pink (Aa) offspring. Punnett squares still apply but phenotype ratios equal genotype ratios.
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