Punnett square calculator with ratios and probabilities

A Punnett square shows every way two parents' alleles can combine in their offspring. Write one parent's alleles along the top and the other's down the side, and fill each box with one from each. Two Tt parents give TT, Tt, Tt and tt: a 1 : 2 : 1 genotype ratio and a 3 : 1 ratio of dominant to recessive phenotypes. This Punnett square calculator is our Genetics Cross Lab: choose a trait and the parents' genotypes, and it draws the square, gives both ratios in lowest terms, and works out the chances for a family.

It opens on two pea plants that are both Tt for height. Open the full Genetics Cross Lab over this page to set up your own cross.

Try it

Genetics Cross Lab

Breed generations with Punnett squares and pedigree charts

Open the simulatorGo to its page

How to use the Punnett square calculator

1. Under Cross, choose Monohybrid (one trait) or Dihybrid (two traits).
2. Pick a Trait. Each one is labelled with how it is inherited: complete dominance (pea height, seed shape and seed colour), a recessive condition (cystic fibrosis), a dominant condition (Huntington's disease), codominance (ABO and MN blood groups, roan cattle), incomplete dominance (snapdragon colour) or sex-linked (haemophilia A, red-green colour blindness, fruit fly eye colour).
3. Choose the Mother's and Father's genotypes from the lists.
4. Read the Punnett square, then the Genotype ratio and Phenotype ratio under Offspring.
5. Press play under Simulate offspring to breed up to 5,000 offspring and watch the real counts settle towards the expected ratio. A chi-squared test says whether any gap could be chance.
6. The exam working panel shows the chance of each outcome and, using Children per couple, the chance that all, none or at least one child shows it. The Family tree view builds a pedigree you extend by choosing who breeds.

The calculator works with its list of real traits rather than letters you type. For a textbook question about any trait with one dominant and one recessive allele, use a dominant trait such as pea height: the square and the ratios are the same, only the letters differ.

How to make a Punnett square

1. Work out each parent's genotype, for example Tt.
2. Split each parent into the alleles its gametes can carry: T or t.
3. Write one parent's gametes along the top and the other's down the side.
4. Fill each box with the allele from its column and the allele from its row.
5. Count the boxes: each one is an equally likely outcome.

This is Tt × Tt, the calculator's opening cross. One box in four is TT, two are Tt and one is tt, so the genotype ratio is 1 : 2 : 1. T is dominant, so three boxes are tall and one is short: a phenotype ratio of 3 : 1, and a 1 in 4 (25%) chance that any one offspring is short.

Try one, then press Mark to check your answer in the Genetics and DNA course.

Worked examples you can check in the calculator

Example 1: a test cross

A tall plant could be TT or Tt. Cross it with a short plant (tt). If it is Tt, the square gives Tt, Tt, tt and tt: a 1 : 1 ratio of tall to short. If it is TT, every offspring is Tt and tall. Short offspring prove the tall parent carries t. Set Mother to Tt and Father to tt to see the 1 : 1.

Example 2: carriers of a recessive condition

Choose cystic fibrosis, with both parents Ff: they carry the recessive allele but are unaffected. The square is the same shape as Tt × Tt, so each child has a 1 in 4 chance of cystic fibrosis (ff), a 1 in 2 chance of being a carrier (Ff) and a 1 in 4 chance of FF. Of the unaffected children, 2 in 3 are carriers.

Each child is a separate event, so chances multiply. With Children per couple set to 4, the calculator gives the chance that none of the four is affected as 0.75⁴ = 0.316, and the chance that at least one is affected as 1 − 0.316 = 0.684.

Example 3: incomplete dominance and codominance

In snapdragons, red (C^R C^R) crossed with white (C^W C^W) gives all pink offspring, because neither allele is dominant and the colours blend. Cross two pink plants and you get 1 red : 2 pink : 1 white. Here the phenotype ratio equals the genotype ratio, because each genotype looks different. Codominance is similar, except both alleles show in full: an L^M L^N person has MN blood, with both M and N antigens.

Example 4: a sex-linked cross

Choose haemophilia A, with a carrier mother (X^H X^h) and an unaffected father (X^H Y). The square gives X^H X^H, X^H X^h, X^H Y and X^h Y. No daughters have haemophilia, though half are carriers. Half of the sons have it, because a son's only X comes from his mother. Overall, each child has a 1 in 4 chance of being a son with haemophilia.

Example 5: a dihybrid cross

Choose Dihybrid, with pea seed shape and seed colour, and make both parents RrYy. Each parent can make four kinds of gamete (RY, Ry, rY, ry), so the square has 4 × 4 = 16 boxes. The phenotype ratio is 9 : 3 : 3 : 1: 9 round yellow, 3 round green, 3 wrinkled yellow and 1 wrinkled green. It is the 3 : 1 for each trait multiplied together, since the two genes are inherited independently.

Try one

Ratios, probabilities and real results

A Punnett square gives probabilities, not promises. A 3 : 1 ratio means each offspring has a 3 in 4 chance of the dominant phenotype. A family of four could easily have no short children, or two. Large numbers get close to the ratio, which is what the Simulate offspring button shows: with 500 offspring the counts land near 375 to 125, but rarely exactly. The chi-squared test checks whether the gap is small enough to be chance.

Common mistakes

• Putting both of a parent's alleles in one box. Each gamete carries one allele per gene, so each row or column label is a single allele.
• Mixing up genotype and phenotype ratios. Tt × Tt is 1 : 2 : 1 by genotype and 3 : 1 by phenotype.
• Thinking the ratio tells you exactly what a family will have. Each child is a fresh 1 in 4, whatever happened before.
• Giving sons two X chromosomes in a sex-linked cross. Sons get Y from their father.
• Writing the dihybrid gametes wrongly. RrYy makes RY, Ry, rY and ry: one allele of each gene in every gamete.

Read next

Biology · Genetics and inheritanceDihybrid cross: Punnett square, 9:3:3:1 ratio, examplesHow to do a dihybrid cross: working out the gametes, the 16-box Punnett square, why it gives 9:3:3:1, other ratios, and testing results with chi-squared.