Punnett Square Calculator
Select each parent's genotype for a single gene to find the expected offspring genotype and phenotype ratios.
How a Punnett square works
A Punnett square predicts the possible genotype combinations of offspring from a genetic cross by combining each parent's possible gametes (sex cells). Each parent contributes one allele per gene to each offspring, and every combination of the two parents' alleles is equally likely.
A heterozygous parent (Aa) produces two types of gametes (A and a) in equal proportion, while a homozygous parent (AA or aa) produces only one type. Combining these possibilities across both parents gives the offspring genotype ratios shown here.
Frequently asked questions
What's the difference between genotype and phenotype?
Genotype is the actual genetic makeup (like AA, Aa, or aa), while phenotype is the observable trait that results from it. In simple dominant/recessive inheritance, both AA and Aa genotypes show the dominant phenotype, while only aa shows the recessive phenotype.
Why does Aa × Aa give a 3:1 phenotype ratio but a 1:2:1 genotype ratio?
The four equally likely genotype outcomes are AA, Aa, Aa, and aa — a 1:2:1 ratio. But since both AA and Aa display the dominant phenotype, three of the four outcomes (AA, Aa, Aa) look the same, giving a 3:1 phenotype ratio even though the genotypes differ.
Does this work for traits with more than two alleles or more complex inheritance?
No — this covers simple Mendelian inheritance for a single gene with complete dominance (one dominant, one recessive allele). Traits involving incomplete dominance, codominance, multiple alleles, or multiple genes need more complex analysis beyond a basic Punnett square.