Punnett square calculator, one gene or two.

A Punnett square calculator shows the possible offspring of two parents for one or two genes. Pick each parent's genotype and it draws the square, lists the gametes, and gives the genotype ratio and the phenotype ratio as ratios, fractions and percent. A dihybrid cross such as AaBb × AaBb fills a 4 × 4 square and gives the well-known 9 : 3 : 3 : 1. This is genetics arithmetic for school and practice. It is not a medical test or a prediction for a real person or family.

How to use the Punnett square calculator

1. Choose One gene or Two genes under Genes.
2. Pick Complete dominance, or Incomplete (blend) if the heterozygote shows something between the two traits.
3. Change the letters if your problem uses others, such as R for round seeds.
4. Set the genotype of Parent 1 and Parent 2 for each gene: homozygous dominant (AA), heterozygous (Aa) or homozygous recessive (aa).
5. Read the square and the tables. Click a phenotype row to light its boxes. Use Download picture or Print the square for homework.

How the square is built

Each parent makes gametes, and each gamete carries one allele of each gene. A parent Aa makes A and a. A parent AaBb makes four kinds: AB, Ab, aB and ab. Write one parent's gametes across the top and the other's down the side. Each box is the offspring you get by combining the gamete in its column with the gamete in its row, so a one-gene cross has 4 boxes and a two-gene cross has 16. A homozygous parent such as AA is shown with its gamete twice (A and A), so every box is counted as one equal chance.

Worked examples

Aa × Aa. The gametes are A and a for both parents, so the boxes are AA, Aa, Aa and aa. The genotype ratio is 1 : 2 : 1. With complete dominance, AA and Aa look the same, so the phenotype ratio is 3 : 1, which is 75% dominant and 25% recessive.

Aa × aa, the test cross. The boxes are Aa, Aa, aa and aa, so genotypes and phenotypes are both 1 : 1, or 50% each. A test cross like this is used to tell whether a dominant-looking parent is AA or Aa.

AaBb × AaBb. The 16 boxes give 9 with both dominant traits, 3 with the first dominant and the second recessive, 3 with the first recessive and the second dominant, and 1 with both recessive. That is 9 : 3 : 3 : 1, or 56.25%, 18.75%, 18.75% and 6.25%. The genotype list has 9 kinds, and AaBb is the most common at 4 of 16, or 25%.

AaBb × aabb. Parent 2 makes only ab, so the four boxes are AaBb, Aabb, aaBb and aabb. The ratio is 1 : 1 : 1 : 1, each 25%.

AABB × aabb. Every box is AaBb, so all offspring show both dominant traits. The ratio is 1 : 0 : 0 : 0 and the calculator shows a single phenotype.

Complete and incomplete dominance

With complete dominance one allele hides the other, so Aa looks like AA and there are two phenotypes per gene. With incomplete dominance the heterozygote is a blend, such as pink flowers from red and white parents. There are then three phenotypes per gene, and the phenotype ratio for Aa × Aa becomes 1 : 2 : 1 (shown largest first as 2 : 1 : 1), the same set of numbers as the genotype ratio.

Fractions, percent and what they mean

Each box has the same chance, so a genotype that fills 4 of 16 boxes has a chance of 4/16, which is 1/4 or 25%. These are chances for each offspring, not a count of how many a particular family will get. Four offspring from Aa × Aa do not have to include exactly one aa.

Limits of the method

• Genes that sit close together on one chromosome are linked and do not sort independently. This calculator assumes they do.
• It covers one or two genes with two alleles each. Traits shaped by many genes, by the environment, or by genes on the sex chromosomes need other methods.
• It says nothing about any real person, pregnancy, family or health condition. For that, speak to a qualified genetic counsellor.

To scale a ratio, try the ratio calculator. For chances written as percents, use the percentage calculator.