Monohybrid Genetic Cross Calculator (Punnett Square)
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/9/2026
Crossing two heterozygous parents (Aa × Aa) gives 25% homozygous dominant offspring (AA), 50% heterozygous (Aa), and 25% homozygous recessive (aa), a phenotypic ratio of 3 dominant to 1 recessive.
Explanation
This calculator reproduces the Punnett square, the classic visual tool invented by Reginald Punnett to predict the genotypic proportions of a cross, for a single gene with two alleles (one dominant, labeled A, the other recessive, labeled a) under complete dominance — the dominant phenotype is expressed as soon as a single A allele is present (AA or Aa), the recessive phenotype only appears if both alleles are recessive (aa). Each parent randomly passes on one of their two alleles to each offspring, with equal probability; combining the two possible alleles from each parent gives the four equally likely combinations of the Punnett square, from which the proportions of each resulting genotype can then be calculated. The Aa × Aa cross (two heterozygous parents) is the most iconic case in Mendelian genetics, producing the famous 3:1 phenotypic ratio that Gregor Mendel first observed in his pea plants. This calculator deliberately limits itself to a single gene with complete dominance — many real traits involve several interacting genes, incomplete dominance, codominance, or are X-linked, more complex cases beyond the scope of this simple model. Distinct from our Hardy-Weinberg equilibrium calculator, which predicts genotypic frequencies across an entire population at equilibrium rather than the offspring of one specific individual cross — and from our Shannon diversity index calculator, which measures how many different species or genotypes coexist in a community rather than the odds of a single cross's outcome.
Example: crossing two heterozygotes (Aa × Aa)
Inputs
Parent 1: Aa. Parent 2: Aa.
Calculation
Each parent passes on A or a with probability 1/2. The 4 possible combinations (AA, Aa, aA, aa) are equally likely: 1 AA, 2 Aa (Aa and aA count together), 1 aa, i.e. 25%, 50%, 25%.
Result
This cross produces 25% AA, 50% Aa, and 25% aa, i.e. 75% dominant phenotype and 25% recessive.
Frequently asked questions
Why do Aa and aA count as the same genotype?
Because the order in which the two alleles are inherited (which comes from the father, which from the mother) has no influence on the offspring's genotype or phenotype: having one A allele and one a allele produces exactly the same heterozygous individual, regardless of which parent each allele came from.
Does this calculation give certainty about each individual offspring?
No, these are theoretical probabilities valid on average across a large number of offspring, not a certain prediction for a given individual. As with a coin flip, getting 3 dominant offspring out of the first 4 born from an Aa × Aa cross doesn't contradict the theoretical 3:1 ratio, which only holds precisely on average over a large sample.
Does this model apply to all hereditary traits?
No, this model assumes a single gene with complete dominance, which describes some traits well (like the pea color Mendel studied) but not all. Many real traits involve several interacting genes, incomplete dominance (an intermediate phenotype), codominance (both traits expressed simultaneously), or are carried on sex chromosomes — mechanisms more complex than this simple model covers.