What this calculator does
A Punnett square calculator for the simplest case in Mendelian genetics: a single-gene, or monohybrid, cross between two parents. Enter each parent’s genotype (AA, Aa or aa) and this works out every possible combination of alleles their offspring can inherit, along with the resulting genotype and phenotype ratios.
The textbook example is crossing two heterozygous parents, Aa × Aa, which produces the well-known 1:2:1 genotype ratio (AA : Aa : aa) and, because A is dominant, a 3:1 phenotype ratio of dominant to recessive traits. This calculator generates that result, and the result for any other pairing of AA, Aa and aa parents, the same way a hand-drawn 2x2 grid would.
The formula
Each parent contributes one of its two alleles at random to each offspring. Pairing every allele from parent 1 against every allele from parent 2 produces four equally likely offspring genotypes (the four cells of the Punnett square). Genotypes are then grouped by phenotype: any genotype containing the dominant allele shows the dominant trait, and only the fully recessive genotype shows the recessive trait.
| Term | Meaning |
|---|---|
| Genotype | The pair of alleles an individual carries for this gene, such as AA, Aa or aa. |
| Phenotype | The observable trait that results from the genotype, dominant or recessive. |
| Complete dominance | The assumption that one copy of the dominant allele is enough to produce the dominant phenotype, with no blending or partial expression. |
| Heterozygous | Carrying two different alleles for the gene (Aa), as opposed to homozygous (AA or aa). |
The inputs explained
| Field | What to enter |
|---|---|
| Parent 1 genotype | The first parent’s genotype for this gene: two dominant alleles, one of each, or two recessive alleles. |
| Parent 2 genotype | The second parent’s genotype, using the same three options. |
When to use it
The classic Aa x Aa cross
Two heterozygous parents produce the 1:2:1 genotype ratio and 3:1 phenotype ratio that shows up in almost every introductory genetics course, because it is the simplest cross where the recessive trait can reappear in the offspring even though both parents show the dominant trait themselves.
A test cross (Aa x aa)
Crossing a heterozygous individual against a homozygous recessive one is a standard technique for revealing whether an individual showing the dominant trait is AA or Aa, since the two genotypes give different, testable offspring ratios.
Confirming a homozygous cross
Crossing two homozygous parents (AA x AA, or aa x aa) should always produce uniform offspring, and running it through the calculator is a quick way to confirm that expectation before moving on to a more complex cross.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
Phenotype ratio for every parent-genotype pairing
The dominant-to-recessive phenotype ratio produced by each possible pairing of parent genotypes.
Questions
What ratio does crossing two Aa parents give?
A 1:2:1 genotype ratio (1 AA : 2 Aa : 1 aa) and, since A is dominant, a 3:1 phenotype ratio of dominant to recessive traits. This is the most commonly cited Punnett square result for exactly that reason.
How is this different from the dihybrid cross calculator on this site?
This one tracks a single gene with two possible alleles. The dihybrid cross calculator tracks two genes at once (such as AaBb x AaBb), which needs a 4x4 grid and produces the more complex 9:3:3:1 ratio.
What if I need more than two genes, or non-standard probabilities?
The multi-gene cross probability calculator handles crosses involving several genes and calculates the probability of a chosen number of recessive traits among the offspring, beyond what a simple grid can show at a glance.
Does this assume complete dominance?
Yes. It assumes one copy of the dominant allele is enough to produce the dominant phenotype. Traits with incomplete dominance or codominance, where heterozygotes show a blended or intermediate phenotype, follow different phenotype groupings than this calculator applies.
For a two-gene cross, see the dihybrid cross calculator. For crosses involving more genes or specific probability questions, use the multi-gene cross probability calculator.