# Punnett square

> A Punnett square is a grid that lists each parent's possible gametes along two sides to predict the expected genotype ratios among offspring of a cross.

- Canonical: https://genetixbay.com/knowledge/glossary/punnett-square.html
- Also written: Punnett squares, Punnett grid, Punnet square, cross diagram
- Glossary category: Genetics and breeding · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

A Punnett square is a simple grid for predicting offspring of a cross. One parent's possible gametes go across the top, the other's down the side, and each cell shows the [genotype](https://genetixbay.com/knowledge/glossary/genotype.md) from that combination. The proportions of cells are the expected proportions of offspring.

## In practice

For one [gene](https://genetixbay.com/knowledge/glossary/gene.md), Aa × Aa gives a 2 × 2 square: one AA, two Aa, one aa. If A is dominant, three of four plants look the same. For two independent genes the square is 4 × 4 and gives the 9:3:3:1 ratio.

Breeders use it to plan how many seeds to pop. If you want a plant [homozygous](https://genetixbay.com/knowledge/glossary/homozygous.md) for two [recessive traits](https://genetixbay.com/knowledge/glossary/recessive.md) from an [F2](https://genetixbay.com/knowledge/glossary/f2.md), only 1 in 16 is expected; to be about 95% sure of getting at least one, you need roughly 47 plants. For three recessives it is 1 in 64 and a few hundred seeds.

## Units and formula

Chance of at least one hit in n plants = 1 - (1 - p)^n, where p is the expected fraction from the square. Solve for n to size a grow-out.

## Don't confuse with

- **Actual results**: the square gives expected ratios; small grows deviate by chance.
- **Quantitative traits**: yield or potency do not fit a Punnett square; they need [heritability](https://genetixbay.com/knowledge/glossary/heritability.md) and population thinking.
- **Linked genes**: genes near each other on a chromosome do not assort freely, so the 9:3:3:1 square overstates recombinants.

## Related terms

[Mendelian inheritance](https://genetixbay.com/knowledge/glossary/mendelian-inheritance.md), [Segregation](https://genetixbay.com/knowledge/glossary/segregation.md), [Dominant](https://genetixbay.com/knowledge/glossary/dominant.md), [Recessive](https://genetixbay.com/knowledge/glossary/recessive.md), [Test cross](https://genetixbay.com/knowledge/glossary/test-cross.md)

## Read more

- [Mendelian genetics for growers, with Punnett squares](https://genetixbay.com/knowledge/breeding/mendelian-genetics-for-growers.md)

## Sources

1. Miko I. 2008. Gregor Mendel and the principles of inheritance. Nature Education 1(1):134. https://www.nature.com/scitable/topicpage/gregor-mendel-and-the-principles-of-inheritance-593/