Punnett Square Calculator
Type two parents’ genotypes and get the full Punnett square, the genotype and phenotype ratios, the chance of each outcome, and a step-by-step explanation you can check your homework against.
Genetics problems go wrong in one of three places: the gametes, the boxes, or the counting. This calculator shows all three, so you can find exactly where your answer split from the right one. It handles one, two, or three genes, incomplete and codominant traits, X-linked (sex-linked) traits like colorblindness, and ABO blood types.
If a problem only says “type A,” the parent could be IᴬIᴬ or Iᴬi. Look for a clue, such as a type O parent or child, which means they carry i.
Try one:
How to use it, and common mistakes
- One letter per gene. A capital letter is the dominant allele and lowercase is recessive. Write every gene as a pair:
Aa,BB,cc. Order doesn’t matter, sobBaAreads the same asAaBb. - Homozygous vs. heterozygous.
AAandaaare homozygous (two of the same).Aais heterozygous. “Pure-breeding” or “true-breeding” in a problem means homozygous. - Genotype vs. phenotype. The genotype is the allele pair. The phenotype is the trait you can see. With complete dominance,
AAandAalook the same, which is why Aa × Aa gives a 1 : 2 : 1 genotype ratio but a 3 : 1 phenotype ratio. - Incomplete vs. codominance. Incomplete dominance blends (red × white snapdragons give pink). Codominance shows both at once (a red-and-white roan coat, or AB blood). Set this per gene under Traits.
- X-linked traits. Sons get their only X from their mother, so they show a recessive X-linked trait far more often than daughters. A heterozygous daughter is a carrier.
- The most common homework mistake is writing gametes with two copies of the same gene (like
Aaas one gamete). Each gamete gets exactly one allele per gene. Check your gametes against the ones along the edges of the square. - Probability, not a promise. Each box is one equally likely outcome for each child. A 1/4 chance doesn’t mean exactly one in four children in a real family.
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