# Locus

> A locus is the fixed position on a chromosome where a particular gene or DNA marker sits; the plural is loci.

- Canonical: https://genetixbay.com/knowledge/glossary/locus.html
- Also written: loci, gene locus, genetic locus
- 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 locus is the fixed physical position on a [chromosome](https://genetixbay.com/knowledge/glossary/chromosome.md) where a [gene](https://genetixbay.com/knowledge/glossary/gene.md) or a DNA marker sits. The plural is loci. Every plant of the species has the same loci in the same order (barring rearrangements); what differs between plants is which [alleles](https://genetixbay.com/knowledge/glossary/allele.md) occupy them.

## In practice

Breeders talk about loci when one position controls a clear trait. The [cannabinoid](https://genetixbay.com/knowledge/glossary/cannabinoid.md) synthase region is often called the B locus, after de Meijer's model of [THC](https://genetixbay.com/knowledge/glossary/thc.md)/CBD inheritance. [Autoflowering](https://genetixbay.com/knowledge/glossary/autoflower.md) behaviour maps to a major locus named Autoflower1 (Toth et al. 2022). DNA tests report results locus by locus: a [genotyping](https://genetixbay.com/knowledge/glossary/genotyping.md) panel might read a few thousand SNP loci and compare them between two cuts.

When a trait is controlled by many loci of small effect, the regions are called [quantitative trait loci](https://genetixbay.com/knowledge/glossary/qtl.md) (QTL). Most yield and vigour traits fall in that group.

## Don't confuse with

- **Gene**: a gene occupies a locus, but a locus can also be a marker with no known function.
- **Allele**: the variant found at the locus.
- **Chromosome**: the whole DNA molecule that carries thousands of loci. Cannabis has 10 chromosome pairs.

## Related terms

[Gene](https://genetixbay.com/knowledge/glossary/gene.md), [Allele](https://genetixbay.com/knowledge/glossary/allele.md), [Chromosome](https://genetixbay.com/knowledge/glossary/chromosome.md), [QTL](https://genetixbay.com/knowledge/glossary/qtl.md), [Linkage](https://genetixbay.com/knowledge/glossary/linkage.md)

## Read more

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

## Sources

1. de Meijer EPM, Bagatta M, Carboni A, et al. 2003. The inheritance of chemical phenotype in Cannabis sativa L. Genetics 163(1):335-346. https://doi.org/10.1093/genetics/163.1.335
2. Toth JA, Stack GM, Carlson CH, Smart LB. 2022. Identification and mapping of major-effect flowering time loci Autoflower1 and Early1 in Cannabis sativa L. Frontiers in Plant Science 13:991680. https://doi.org/10.3389/fpls.2022.991680