Punnett square calculator with genotype and phenotype ratios
This Punnett square calculator crosses two parents gene by gene: it lists the gametes, fills the grid and counts genotypes and phenotypes, as OpenStax Biology describes. Example: Aa × Aa gives the genotypes 1 AA : 2 Aa : 1 aa and the phenotypes 3 : 1.
How it is calculated
A Punnett square combines every gamete of one parent with every gamete of the other. OpenStax explains it like this: the possible alleles of one parent go along the top, those of the other parent down the side, and each square is one equally likely offspring. Capital letters stand for dominant alleles, small letters for recessive ones.
Monohybrid cross: 1 : 2 : 1 and 3 : 1
For Mendel’s seed colour (Y = yellow, y = green), Yy × Yy gives the four squares YY, Yy, Yy and yy. Genotypes: 1 YY : 2 Yy : 1 yy. Because YY and Yy both look yellow, the phenotypes are 3 yellow : 1 green, i.e. 75 % and 25 %. A test cross with a homozygous recessive partner (Yy × yy) gives 1 : 1, i.e. 50 % each – this is how breeders find out whether a dominant-looking plant is YY or Yy.
Dihybrid cross: 9 : 3 : 3 : 1
| Cross | Squares | Phenotype ratio |
|---|---|---|
| Aa × Aa | 4 | 3 : 1 |
| Aa × aa | 4 | 1 : 1 |
| AA × aa | 4 | all Aa |
| AaBb × AaBb | 16 | 9 : 3 : 3 : 1 |
| AaBb × aabb | 16 | 1 : 1 : 1 : 1 |
With two genes each parent forms four gametes (AB, Ab, aB, ab), so the grid has 16 squares. OpenStax shows for YyRr × YyRr: 9 round/yellow : 3 round/green : 3 wrinkled/yellow : 1 wrinkled/green – 9 of 16 squares are 56.25 %. This only holds if the genes are inherited independently; genes that lie close together on one chromosome break the 9 : 3 : 3 : 1 rule. Whether counted offspring fit an expected ratio is checked with the chi-square test calculator; how to work with the shares is shown in the probability calculator.
Frequently asked questions
What does a Punnett square show?
All equally likely combinations of the parents’ gametes. Each square is one possible offspring genotype; with four squares each one stands for 25 %.
What is the ratio of Aa × Aa?
Genotypes 1 AA : 2 Aa : 1 aa, phenotypes 3 : 1 – 75 % show the dominant trait, 25 % the recessive one.
Why is the dihybrid ratio 9 : 3 : 3 : 1?
Each gene splits 3 : 1 independently. 3 : 1 times 3 : 1 gives 9 : 3 : 3 : 1 on 16 squares.
What is a test cross?
Crossing an individual with the dominant phenotype with a homozygous recessive one (aa). All offspring dominant means AA; a 1 : 1 split means Aa.
Can I calculate three genes?
Yes, up to three letter pairs such as AaBbCc. The grid then has up to 8 × 8 = 64 squares; AaBbCc × AaBbCc gives 27 : 9 : 9 : 9 : 3 : 3 : 3 : 1.
Does it work for blood types or incomplete dominance?
Not directly: the calculator assumes complete dominance. For ABO blood groups with codominance there is a separate blood type calculator.
Sources and legal basis
- OpenStax Biology 2e, 12.2 Characteristics and Traits: Punnett square, monohybrid cross Yy × Yy → genotypes 1 YY : 2 Yy : 1 yy, phenotypes 3 yellow : 1 green; test cross with a homozygous recessive partner (read 10.10.2026)
- OpenStax Biology 2e, 12.3 Laws of Inheritance: dihybrid cross YyRr × YyRr → 9 round/yellow : 3 round/green : 3 wrinkled/yellow : 1 wrinkled/green; independent assortment, not valid for linked genes (read 10.10.2026)
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