# Punnett square

A grid that shows all the possible combinations of alleles from two parents and how likely each is.

Biology (school) · Part 05: Genes and inheritance · Card 7 of 11 · Last updated 2026-10-19

## Analogy

A multiplication table, but for alleles.

## Example

**Tt x Tt cross**

Offspring: TT, Tt, Tt, tt
Tall (T dominant): 3 out of 4
Short (tt): 1 out of 4
Ratio 3 : 1

## In practice

It can show the chance that two carriers of cystic fibrosis have an affected child.

## Practice

### Flashcards

- **What ratio comes from crossing two heterozygous parents?** 3 dominant : 1 recessive.

### Interview questions

**Explain how a Punnett square is used to predict the outcome of a genetic cross.**

- Write the parents' alleles along the top and side
- Fill in each box with the allele pairs from both parents
- Work out the genotype of each box
- Count boxes to get probabilities or a ratio

## Questions people ask

### How do you fill in a Punnett square?

Draw a two by two grid. Write one parent's two alleles above the columns and the other parent's alleles beside the rows. In each box, combine the allele from the column with the allele from the row. Then count the genotypes and phenotypes. Probabilities are shown as fractions or ratios.

### Does a Punnett square predict exactly what will happen?

No. It shows probabilities, not certainties. A 1 in 4 chance does not mean that exactly one of four children will have the trait. Each child has the same chance each time, like flipping a coin. Over many births the results get closer to the predicted ratio.

## Related concepts

- [Genotype and phenotype](https://studyvisually.com/learn/science/biology-school/biology-school-part-5/biology-school-part-5-concept-6)
- [Allele](https://studyvisually.com/learn/science/biology-school/biology-school-part-5/biology-school-part-5-concept-4)
- [Dominant and recessive alleles](https://studyvisually.com/learn/science/biology-school/biology-school-part-5/biology-school-part-5-concept-5)
- [Sex chromosomes](https://studyvisually.com/learn/science/biology-school/biology-school-part-5/biology-school-part-5-concept-10)

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