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 one of the most widely used tools for teaching Mendelian inheritance and predicting the probability of offspring genotypes from simple genetic crosses.
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).
Calculator scope: This Punnett square calculator models one gene with two alleles under complete dominance. It does not model codominance, incomplete dominance, sex-linked inheritance, linked genes or polygenic traits.
An individual carries two alleles for a gene, usually one inherited from each parent. During the formation of eggs or sperm, these two alleles separate, so each reproductive cell carries only one allele. At fertilization, the offspring receives one allele from each parent.
A Punnett square displays every possible combination of the alleles that each parent can contribute. For example, a parent with genotype Aa can pass on either A or a.
A genotype is the pair of alleles an individual carries. A phenotype is the observable trait produced by that genotype.
Under complete dominance, a single dominant allele is enough to produce the dominant phenotype. This means both AA and Aa produce the dominant phenotype. The recessive phenotype appears only when the genotype is aa.
Punnett square results describe the probability for each individual offspring. They do not guarantee that a family or group of offspring will exactly match the predicted ratio.
For example, a 25% probability does not mean that exactly every fourth child will inherit the genotype. Each conception is an independent event, so the same outcome can occur several times in a row.
Observed ratios usually become closer to the predicted probabilities as the number of offspring increases, but small families may differ substantially from the expected ratio by chance.
A basic Punnett square is most useful for simple inheritance involving one gene, two alleles and complete dominance. Many real traits follow more complex inheritance patterns.
The heterozygous phenotype is intermediate between the two homozygous phenotypes rather than matching the dominant allele.
Both alleles are expressed in the heterozygous individual, as occurs in the AB blood group.
Traits such as height and skin pigmentation are influenced by many genes and cannot be represented accurately by a single-gene Punnett square.
Genes located close together on the same chromosome may be inherited together more often than expected under independent assortment.
Mitochondrial DNA is usually inherited through the maternal line and does not follow the same pattern as a standard autosomal cross.
Nutrition, temperature, disease and other environmental factors can influence how a genotype appears as an observable phenotype.
This calculator should therefore be used for simple Mendelian crosses, not as a predictor for complex human traits or medical outcomes.
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.
Explore these authoritative resources for more information about genetics, inheritance patterns, Punnett squares and Mendelian inheritance.
Definitions and explanations of common genetics and genomics terms from the National Human Genome Research Institute.
An overview of autosomal dominant, autosomal recessive, X-linked and mitochondrial inheritance patterns.
A detailed introduction to Mendelian inheritance, dominant and recessive alleles, and common single-gene inheritance patterns.
An interactive explanation of genetic probability, gamete formation and how Punnett squares represent possible offspring combinations.
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