Punnett Square Ratio Calculator
A Punnett square is the classic tool for predicting how a single gene is inherited, and this calculator builds one for you. Pick the genotype of each parent for one gene with two alleles, a dominant allele written as a capital letter and a recessive allele written in lowercase, and the page lists every offspring combination, then counts the genotype and phenotype ratios. Each parent passes one of its two alleles to each offspring, so the square has four equally likely cells. The most famous case is a cross between two heterozygotes, Aa by Aa, which yields AA, Aa, aA and aa: a genotype ratio of one to two to one and a phenotype ratio of three dominant to one recessive, because a single dominant allele is enough to show the dominant trait. That means 75 percent of offspring are expected to show the dominant phenotype and 25 percent the recessive one. These figures are probabilities over many offspring, not guarantees for any one, much as a handful of coin tosses need not split evenly. Use it for homework, breeding predictions or genetics revision. Every count is computed deterministically by enumerating the allele combinations, never estimated, with the method and a worked example shown below for verification.
A monohybrid cross enumerates four allele pairs and counts them: Aa x Aa gives 1:2:1 genotype and 3:1 phenotype. The dominant trait is expected in 75.00% of offspring and the recessive in 25.00%.
How the ratio is found
offspring = each allele of parent 1 paired with each allele of parent 2
four equally likely combinations per cross
genotype ratio = counts of AA, Aa and aa
phenotype: AA and Aa are dominant, aa is recessive
Pairing the two alleles of one parent with the two of the other gives four cells. Counting the genotypes and grouping the dominant ones produces the phenotype ratio.
Worked example
Cross two heterozygous parents, Aa by Aa.
- Parent 1 alleles A and a; parent 2 alleles A and a
- Combinations: AA, Aa, aA (counted as Aa), aa
- Genotype counts: 1 AA, 2 Aa, 1 aa, a 1:2:1 ratio
- Dominant (AA + Aa) = 3, recessive (aa) = 1, a 3:1 ratio
- Dominant phenotype = 3 / 4 = 75.00%
The dominant trait appears in 75.00 percent of offspring. These are the calculator's default inputs, so the result above matches the widget exactly.
Common monohybrid crosses
Expected phenotype ratios for one gene.
| Cross | Phenotype ratio |
|---|---|
| AA x AA | All dominant |
| Aa x Aa | 3 dominant : 1 recessive |
| Aa x aa | 1 dominant : 1 recessive |
| aa x aa | All recessive |
Genetics and inheritance concepts: US Geological Survey (USGS).
Punnett square ratio calculator: frequently asked questions
What is a monohybrid cross?
A monohybrid cross follows a single gene with two alleles, one dominant and one recessive. Each parent passes one allele to each offspring, and the Punnett square lists every combination so you can count the genotype and phenotype proportions.
Why does Aa by Aa give a 3 to 1 ratio?
Crossing two heterozygotes produces four equally likely combinations: AA, Aa, aA and aa. Three of the four carry at least one dominant allele and show the dominant trait, while one is recessive, giving a 3 to 1 phenotype ratio and a 1 to 2 to 1 genotype ratio.
What is the difference between genotype and phenotype ratio?
The genotype ratio counts the exact allele pairs, such as 1 AA to 2 Aa to 1 aa. The phenotype ratio counts visible traits, grouping AA and Aa together as dominant because one dominant allele is enough to show the trait, giving 3 dominant to 1 recessive.
Are these guaranteed outcomes?
No. The ratios are probabilities over many offspring, not certainties for any single one. With a small number of offspring the actual counts can differ from the predicted ratio, just as a few coin tosses may not be exactly half heads.
Is the result computed automatically?
Yes. The page enumerates the four allele combinations from the two parent genotypes deterministically and counts them. No value is estimated or hard-coded, so changing either parent updates the ratios instantly.
Official sources
- Genetics and inheritance concepts: US Geological Survey (USGS). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.