# Homozygous

> A plant is homozygous at a locus when it carries two identical alleles there (AA or aa), so it passes the same allele to every offspring.

- Canonical: https://genetixbay.com/knowledge/glossary/homozygous.html
- Also written: homozygote, homozygosity, homozygotes
- 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 plant is homozygous at a [locus](https://genetixbay.com/knowledge/glossary/locus.md) when both copies of the [gene](https://genetixbay.com/knowledge/glossary/gene.md) are the same [allele](https://genetixbay.com/knowledge/glossary/allele.md), for example AA or aa. Every gamete it makes carries that allele, so for that trait it breeds true. A line can be homozygous at one locus and [heterozygous](https://genetixbay.com/knowledge/glossary/heterozygous.md) at thousands of others.

## In practice

"Stable" in breeder talk means "homozygous for the traits that matter". [Inbreeding](https://genetixbay.com/knowledge/glossary/inbred-line.md) (sib crossing, backcrossing, [selfing](https://genetixbay.com/knowledge/glossary/selfing.md)) raises homozygosity generation by generation. One generation of selfing removes, on average, half of the remaining heterozygosity; full-sib mating removes it more slowly.

A plant that is homozygous for its key traits is valuable as a parent: cross it to anything and it donates the same alleles every time, which makes its offspring predictable. That is the logic behind IBLs and inbred parents for [F1 seed](https://genetixbay.com/knowledge/glossary/f1.md). The price is that high homozygosity across the whole genome also exposes deleterious recessives, which is [inbreeding depression](https://genetixbay.com/knowledge/glossary/inbreeding-depression.md).

## Units and formula

Expected heterozygosity after n generations of selfing, starting from a fully heterozygous locus: (1/2)^n. After S1: 50%; after S3: 12.5%; after S6: about 1.6%.

## Don't confuse with

- **Heterozygous**: two different alleles at the locus.
- **Uniform**: a line can look uniform while carrying hidden heterozygosity at many loci.

## Related terms

[Heterozygous](https://genetixbay.com/knowledge/glossary/heterozygous.md), [Allele](https://genetixbay.com/knowledge/glossary/allele.md), [True breeding](https://genetixbay.com/knowledge/glossary/true-breeding.md), [IBL](https://genetixbay.com/knowledge/glossary/ibl.md), [Inbred line](https://genetixbay.com/knowledge/glossary/inbred-line.md)

## Read more

- [Backcrossing, IBLs and stabilisation](https://genetixbay.com/knowledge/breeding/backcrossing-ibl-stabilisation.md)
- [Mendelian genetics for growers, with Punnett squares](https://genetixbay.com/knowledge/breeding/mendelian-genetics-for-growers.md)

## Sources

1. Allard RW. 1999. Principles of Plant Breeding, 2nd edition. John Wiley & Sons, New York. ISBN 978-0-471-02309-0
2. Falconer DS, Mackay TFC. 1996. Introduction to Quantitative Genetics, 4th edition. Longman, Harlow. ISBN 978-0-582-24302-6