Punnett Square Calculator

A Punnett square is a compact model for predicting how alleles may combine in offspring. This calculator handles a monohybrid cross: one gene, two allele forms, and diploid parents that each contribute one allele. The uppercase A represents a dominant allele and lowercase a represents a recessive allele. Select AA, Aa, or aa for each parent and the tool lists all four equally likely combinations produced by their two allele slots. Genotype describes the allele pair an offspring inherits. AA is homozygous dominant, Aa is heterozygous, and aa is homozygous recessive. Under a simple complete-dominance model, both AA and Aa show the dominant phenotype, while only aa shows the recessive phenotype. A classic Aa by Aa cross therefore predicts a one-to-two-to-one genotype ratio and a three-to-one phenotype ratio. Percentages represent theoretical probabilities for each independent offspring, not a promise that a small litter or family will contain those exact proportions. The model rests on Mendel's principles of segregation and random fertilization. Each parent passes one of its two alleles, and every listed gamete combination is treated as equally probable. Real inheritance can be more complex. Incomplete dominance, codominance, sex linkage, lethal alleles, linkage between genes, variable penetrance, environmental effects, and biased gamete viability can all change expected phenotypes or frequencies. Those situations need a model tailored to the trait rather than this basic dominant-recessive square. Use this calculator to teach inheritance, check homework, plan a simple breeding cross, or quickly compare parental genotypes. Begin by defining which allele is dominant from reliable biological evidence; capitalization alone does not make an allele biologically dominant. If the parents' genotypes are uncertain, run each plausible cross and combine the predictions with pedigree, test-cross, or molecular evidence. For observed offspring, remember that random sampling causes variation, especially when sample sizes are small. Larger samples usually approach theoretical expectations more closely.