Biology / Genetics
Punnett Square Calculator (Monohybrid and Dihybrid Crosses)
Cross two parents' genotypes, for one, two, or three genes, and see the Punnett square, every offspring genotype with its chance, and the phenotype ratio with complete dominance.
Punnett Square Calculator (Monohybrid and Dihybrid Crosses): Each parent's gametes take one allele from every gene, in every combination: AaBb makes AB, Ab, aB, and ab. The square pairs each gamete of one parent with each of the other, and the offspring are counted. With capital letters dominant, Aa × Aa gives AA : Aa : aa as 1 : 2 : 1 and a 3 : 1 phenotype ratio; AaBb × AaBb gives 16 boxes and the 9 : 3 : 3 : 1 ratio. Runs 100% locally in your browser with zero server file uploads.
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| AB | Ab | aB | ab | |
|---|---|---|---|---|
| AB | AABB | AABb | AaBB | AaBb |
| Ab | AABb | AAbb | AaBb | Aabb |
| aB | AaBB | AaBb | aaBB | aaBb |
| ab | AaBb | Aabb | aaBb | aabb |
| Genotype | Chance |
|---|---|
| AaBb | 4/16 (25%) |
| AABb | 2/16 (12.5%) |
| AaBB | 2/16 (12.5%) |
| Aabb | 2/16 (12.5%) |
| aaBb | 2/16 (12.5%) |
| AABB | 1/16 (6.25%) |
| AAbb | 1/16 (6.25%) |
| aaBB | 1/16 (6.25%) |
| aabb | 1/16 (6.25%) |
| Phenotype (dominant written as A_) | Chance |
|---|---|
| A_ B_ | 9/16 (56.25%) |
| A_ bb | 3/16 (18.75%) |
| aa B_ | 3/16 (18.75%) |
| aa bb | 1/16 (6.25%) |
Each parent passes on one allele of each gene, chosen at random and independently for different genes. The square lists every combination of the parents' gametes; with complete dominance, a capital letter hides a small one, so Aa looks like AA. Two heterozygous parents give the classic 3 : 1 ratio for one gene and 9 : 3 : 3 : 1 for two. Linked genes, incomplete dominance, and sex-linked traits follow other ratios.
Worked examples
Two brown-eyed parents who both carry blue (Bb × Bb): each child has a 1 in 4 chance of blue eyes (bb), in this simplified one-gene model. A tall heterozygous pea (Tt) crossed with a short one (tt): half the offspring tall, half short.
Counting the combinations is the same mathematics as in the permutation and combination calculator.
Beyond simple dominance
Incomplete dominance blends traits, so red × white snapdragons give pink; codominance shows both, as in the AB blood group. Genes close together on a chromosome are inherited together more often than the square predicts.
To look at the DNA sequence behind a gene, use the DNA sequence tools.
How to use it
- Type each parent's genotype, one letter pair per gene, such as Aa or AaBb.
- Read the Punnett square of all gamete combinations.
- Read the genotype chances and the phenotype ratio.
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Related tools
Frequently asked questions
What does A_ mean?
Any genotype with at least one dominant A, so AA or Aa; they show the same trait under complete dominance.
What is a test cross?
Crossing with a homozygous recessive (aa): an Aa parent then gives half Aa and half aa offspring, revealing its genotype.
Why do real families not match the ratios?
Ratios are probabilities for each child, like coin flips; small families often differ from them, and many traits depend on several genes and the environment.
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