How this instrument works
A Punnett square is a simple grid method, devised by geneticist Reginald Punnett in the early 1900s, for predicting the genotypes offspring can inherit from two parents at a single gene. Each parent's two alleles are written along one edge of a 2×2 grid, and each of the four interior boxes shows one possible combination — the genotype of a hypothetical offspring that received one specific allele from each parent. Because each combination in the grid is equally likely, counting how often each genotype appears across the four boxes gives the expected offspring ratio directly.
For a monohybrid cross — one gene, dominant allele A and recessive allele a — the three possible parent genotypes are AA (homozygous dominant), Aa (heterozygous) and aa (homozygous recessive). Cross two heterozygotes, Aa × Aa, and the classic result appears: each parent contributes A or a with equal probability, giving four equally likely combinations — AA, Aa, aA and aa — which simplify to a 1 AA : 2 Aa : 1 aa genotype ratio, the same 3-to-1 dominant-to-recessive phenotype split Gregor Mendel first documented in his 1866 pea-plant experiments.
The other two crosses this instrument can compute are just as informative in their own way. AA × aa — homozygous dominant crossed with homozygous recessive — necessarily produces all-heterozygous offspring, since every offspring gets a guaranteed A from one parent and a guaranteed a from the other. Aa × aa, the classic 'testcross,' produces a 1:1 split of Aa to aa offspring, and geneticists have historically used exactly this cross to determine an organism's unknown genotype: if an individual showing the dominant trait produces roughly half recessive-trait offspring when crossed with a known homozygous recessive, it confirms the tested parent was heterozygous.
- Select the first parent's genotype from Parent 1 genotype — AA, Aa, or aa.
- Select the second parent's genotype from Parent 2 genotype using the same three options.
- Read Offspring genotype ratio beneath both fields — it updates instantly for any combination you choose.
- Try Aa × aa if you want to see a testcross, the classic method for revealing whether a dominant-trait individual is homozygous or heterozygous.
Worked example — Aa crossed with Aa
Select Aa for Parent 1 genotype and Aa for Parent 2 genotype. Offspring genotype ratio reads 1 AA : 2 Aa : 1 aa — the classic heterozygote-by-heterozygote result, since each parent contributes A or a with equal probability, giving four equally likely combinations (AA, Aa, aA, aa) that simplify to that ratio.
In phenotype terms, since A is dominant, three of every four offspring (AA, Aa and Aa) show the dominant trait and one in four (aa) shows the recessive trait — the 3:1 phenotypic ratio Mendel first observed and that underlies this genotype ratio.
Questions
Why does Aa × Aa give a 1:2:1 ratio instead of 1:1:1?
Because there are two different ways to produce a heterozygous Aa offspring — A from parent 1 with a from parent 2, or a from parent 1 with A from parent 2 — while there's only one way each to produce AA (A from both) or aa (a from both). Counting all four equally likely combinations in the Punnett square gives one AA, two Aa (from the two different parent-of-origin combinations) and one aa, which is the 1:2:1 ratio.
What offspring does AA crossed with aa produce?
All offspring are Aa. Every offspring necessarily receives the dominant A allele from the AA parent and the recessive a allele from the aa parent, so all four boxes of the Punnett square read the same genotype — a useful reminder that a Punnett square ratio isn't always a mix; a fully homozygous cross gives a single guaranteed outcome.
What is a testcross, and why is Aa × aa used for it?
A testcross breeds an individual showing a dominant trait, but whose genotype (AA or Aa) is unknown, against a known homozygous recessive (aa) partner. If the unknown parent is actually AA, every offspring will show the dominant trait; if it's Aa, roughly half the offspring will show the recessive trait, matching the 1 Aa : 1 aa ratio this instrument returns for the Aa × aa selection. Historically, this was the standard way to determine genotype before DNA testing existed.
Does this calculator handle two genes at once, like AaBb crosses?
No — this instrument covers a single gene (monohybrid cross) with up to three genotypes per parent. For three genes crossed simultaneously, this site's trihybrid cross Punnett square calculator extends the same underlying logic — each gene's independent 3:1 or 1:1 split multiplied together — to a full eight-phenotype-class result.
Why do geneticists still use Punnett squares when software can do genetics faster?
Because for a single gene or two, the grid is fast to draw by hand, requires no software, and makes the underlying probability logic — every combination equally likely — visually obvious in a way a computed ratio alone doesn't. It remains the standard teaching tool for exactly that reason, even though more genes or more complex inheritance patterns, like linked genes, call for other methods.