# Test cross

> A test cross mates a plant of unknown genotype with one that is homozygous recessive for a trait, so the offspring ratios reveal what the first plant carries.

- Canonical: https://genetixbay.com/knowledge/glossary/test-cross.html
- Also written: testcross, test-crossing, test crosses, progeny test cross
- 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 test cross is a cross used to find out a plant's [genotype](https://genetixbay.com/knowledge/glossary/genotype.md). The unknown plant is crossed with a tester that is [homozygous](https://genetixbay.com/knowledge/glossary/homozygous.md) [recessive](https://genetixbay.com/knowledge/glossary/recessive.md) (aa) for the trait. If all offspring show the [dominant](https://genetixbay.com/knowledge/glossary/dominant.md) trait, the unknown is probably AA. If roughly half show the recessive, it is Aa.

## In practice

Breeders use test crosses to check whether a plant carries a hidden recessive before using it widely. Example: to learn whether a [photoperiod](https://genetixbay.com/knowledge/glossary/photoperiod.md) male carries the [autoflowering](https://genetixbay.com/knowledge/glossary/autoflower.md) allele, cross him to an autoflowering female (homozygous recessive). Any autoflowering offspring prove he is a carrier.

The term is also used more loosely for any small trial cross made to judge a parent, especially a male, by its offspring. That is closer to a [progeny test](https://genetixbay.com/knowledge/glossary/progeny-test.md). Either way, the more offspring, the firmer the answer: in a true test cross of an Aa plant, about 1 in 32 litters of 5 [seedlings](https://genetixbay.com/knowledge/glossary/seedling.md) would show no recessives purely by chance.

## Units and formula

AA × aa: 100% dominant phenotype. Aa × aa: 1:1 dominant to recessive. Chance an Aa parent shows no recessive offspring in n seedlings: (1/2)^n.

## Don't confuse with

- **Test grow**: growing out seed to evaluate a line, not a genetic test of one locus.
- **Backcross**: crossing to a parent to recover its traits, not to diagnose a genotype.

## Related terms

[Recessive](https://genetixbay.com/knowledge/glossary/recessive.md), [Heterozygous](https://genetixbay.com/knowledge/glossary/heterozygous.md), [Progeny test](https://genetixbay.com/knowledge/glossary/progeny-test.md), [Punnett square](https://genetixbay.com/knowledge/glossary/punnett-square.md), [Male selection](https://genetixbay.com/knowledge/glossary/male-selection.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/
2. Allard RW. 1999. Principles of Plant Breeding, 2nd edition. John Wiley & Sons, New York. ISBN 978-0-471-02309-0