Punnett Square Calculator
Cross two parent genotypes to see the Punnett square plus the genotype and phenotype ratios for a single gene.
A monohybrid cross for one gene. Enter each parent's genotype as two letters — the same letter in upper and lower case, like Aa. The capital letter is the dominant allele. Cross Aa × Aa and you get the classic 1:2:1 genotype and 3:1 phenotype split.
| A | a | |
| A | AA | Aa |
| a | Aa | aa |
Genotype ratio
1 AA : 2 Aa : 1 aa
Phenotype ratio (dominant : recessive)
3 : 1
How it works
A Punnett square lays out every way two parents' alleles can combine. For one gene, each parent passes on one of their two alleles, so a 2×2 grid captures all four equally likely offspring.
Enter each genotype as two letters using the same letter in upper and lower case — the capital letter is the dominant allele. Cross Aa with Aa and the grid fills with AA, Aa, Aa, and aa.
From those four boxes you read off the genotype ratio (1 AA : 2 Aa : 1 aa) and, since one dominant allele is enough to show the dominant trait, the phenotype ratio of 3 dominant : 1 recessive.
Frequently asked questions
What does a capital versus lowercase letter mean here?
The capital letter is the dominant allele and the lowercase is recessive. An offspring shows the recessive trait only when it inherits two lowercase alleles, like aa.
Why is Aa × Aa always 3:1 for the phenotype?
Three of the four boxes (AA, Aa, Aa) carry at least one dominant allele, so they all show the dominant trait. Only aa shows the recessive one, giving the familiar 3:1 split.
Can it do dihybrid crosses with two genes?
Not this one — it handles a single gene at a time. A two-gene cross needs a 4×4 grid with sixteen boxes, which is a different tool.