How this calculator works
Two independently assorting loci give each double-heterozygous parent four gamete combinations. This page enumerates 16 allele pairings and groups them into up to nine unordered genotypes. Phenotypes assume complete dominance at each locus. Linked genes, recombination fractions, viability selection and epistasis are outside this independent model.
Formula
Each gamete pairing has probability 1/(2^loci × 2^loci). Genotype probability = matching pairings/total pairings. Independent-locus probabilities multiply.
Worked example
AaBb crossed with AaBb gives both-dominant phenotype 9/16 and both-recessive phenotype 1/16. AABB crossed with aabb produces AaBb with probability one.
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Frequently asked questions
When is the familiar 9:3:3:1 ratio valid?
It is the phenotype ratio for AaBb × AaBb when loci assort independently and each shows complete dominance.
Can linked loci use this result?
Only when independent assortment is a suitable approximation. Close linkage changes gamete probabilities.
Why are genotype and phenotype tables different?
AA and Aa share a dominant phenotype but remain distinct genotypes. The genotype table preserves that distinction.