Free Punnett Square Maker & Calculator

Write down each parent's genotype, then split it into the gametes that parent can make. A parent who is Aa makes A and a in equal numbers; a parent who is AaBb makes AB, Ab, aB and ab. One parent's gametes go across the top of the square and the other parent's down the side.

Fill each box by combining the gamete at the top of its column with the gamete at the start of its row. Every box is equally likely, so a 2×2 square gives four quarters, a 4×4 square sixteen sixteenths, and an 8×8 square sixty-four sixty-fourths.

Finally, count. Group the boxes by genotype for the genotype ratio, then by what the offspring look like for the phenotype ratio. This maker does the counting for you and shows each result as a ratio, a fraction and a percentage.

A monohybrid cross follows one gene. Crossing two heterozygotes, Aa × Aa, gives the classic 3 : 1 phenotype ratio that Mendel saw in his peas. A test cross, Aa × aa, gives 1 : 1 and shows whether a dominant-looking parent carries the recessive allele.

A dihybrid cross follows two genes at once. Because the genes assort independently, RrYy × RrYy gives 9 : 3 : 3 : 1 — nine round yellow seeds for every three round green, three wrinkled yellow and one wrinkled green. A trihybrid cross adds a third gene and gives 27 : 9 : 9 : 9 : 3 : 3 : 3 : 1 across a 64-box square.

With incomplete dominance the heterozygote is in between: red and white snapdragons give pink offspring, and crossing two pinks gives red, pink and white in a 1 : 2 : 1 ratio. With codominance both alleles show at once, as in roan cattle with both red and white hairs.

Blood type has three alleles: Iᴬ and Iᴮ are codominant and both are dominant to i, so a type A parent (Iᴬi) and a type B parent (Iᴮi) can have children of all four blood types. X-linked traits such as color blindness sit on the X chromosome, which is why each son of a carrier mother has a one-in-two chance of the trait while, if the father is unaffected, her daughters are at most carriers.

A Punnett square predicts the children of one couple. To follow a trait through a whole family and work out how it is inherited, draw a pedigree with the pedigree chart maker — it numbers every generation and checks which modes of inheritance fit.