Punnett square calculator
Enter your values. The answer and the full worked solution update as you type.
Result
| AaBb × AaBb | AB | Ab | aB | ab |
|---|---|---|---|---|
| AB | AABB | AABb | AaBB | AaBb |
| Ab | AABb | AAbb | AaBb | Aabb |
| aB | AaBB | AaBb | aaBB | aaBb |
| ab | AaBb | Aabb | aaBb | aabb |
Phenotype ratios assume complete dominance and genes that assort independently (no linkage).
Draw the Punnett square for a cross between two parents with one, two or three genes, such as Aa × Aa, AaBb × AaBb or AaBbCc × aabbcc. The calculator lists the gametes of each parent, fills in every box, counts the genotypes and phenotypes, and gives each as a ratio, a fraction and a percentage. For crosses with more than one gene it checks the answer with the product rule.
Step-by-step solution
Gametes of each parent
Fill in the Punnett square
Genotype probabilities
Phenotypes (complete dominance)
Check with the product rule
Key formulas
- Monohybrid cross (Aa × Aa)
- genotypes 1 : 2 : 1, phenotypes 3 : 1
- Dihybrid cross
- phenotypes 9 : 3 : 3 : 1
- Hardy–Weinberg
- p + q = 1, p² + 2pq + q² = 1
- Independent genes
- multiply the probabilities
How to make a Punnett square
- Write down the genotype of each parent, with a capital letter for the dominant allele and a small letter for the recessive one, for example Aa.
- List the gametes. Each gamete carries one allele of every gene: Aa gives A and a; AaBb gives AB, Ab, aB and ab.
- Draw a grid with the gametes of one parent along the top and those of the other parent down the side, and combine them in every box.
- Count the genotypes and divide by the number of boxes to get their probabilities.
- Group the genotypes by phenotype. With complete dominance, every genotype with at least one capital letter for a gene shows the dominant trait.
Questions and answers
Why is the dihybrid ratio 9:3:3:1?
For AaBb × AaBb each gene on its own gives 3/4 dominant and 1/4 recessive. With independent assortment the probabilities multiply: 3/4·3/4 = 9/16 dominant for both traits, 3/4·1/4 = 3/16 for each mixed phenotype and 1/4·1/4 = 1/16 recessive for both.
What is a test cross?
A cross with a homozygous recessive parent, such as A_ × aa. If any offspring show the recessive trait, the unknown parent must be Aa. For AaBb × aabb the four phenotypes appear in a 1:1:1:1 ratio.
What does A_ mean?
The underscore stands for either allele: A_ means AA or Aa. With complete dominance both show the dominant phenotype.
Does it work for incomplete dominance or sex-linked genes?
The genotype ratios are correct for genes that are not linked. The phenotype ratios assume complete dominance; with incomplete dominance or codominance every genotype has its own phenotype, so read the genotype ratio instead. Sex-linked and linked genes are not modelled.
How do I enter the genotypes?
Write each gene as two letters, gene after gene, for example AaBb. The second parent must have the same genes, in any order, and spaces are ignored, so Bb Aa also works. Up to three genes are supported.
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