# QTL

> A QTL (quantitative trait locus) is a region of the genome statistically linked to variation in a quantitative trait such as yield or flowering time.

- Canonical: https://genetixbay.com/knowledge/glossary/qtl.html
- Also written: quantitative trait locus, quantitative trait loci, QTLs, QTL mapping
- 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 QTL (quantitative trait locus) is a [stretch](https://genetixbay.com/knowledge/glossary/stretch.md) of the genome whose variants are associated with differences in a [quantitative trait](https://genetixbay.com/knowledge/glossary/quantitative-trait.md). A QTL may contain one causal [gene](https://genetixbay.com/knowledge/glossary/gene.md) or several; mapping often narrows the location long before anyone knows which gene is responsible.

## In practice

QTL are found by growing a [segregating](https://genetixbay.com/knowledge/glossary/segregation.md) population (an [F2](https://genetixbay.com/knowledge/glossary/f2.md) or similar), measuring the trait, [genotyping](https://genetixbay.com/knowledge/glossary/genotyping.md) each plant at many markers, and testing which markers track the trait. Genome-wide association studies (GWAS) do the same across unrelated plants. In cannabis, studies have mapped regions for flowering time and sex expression in hemp (Petit et al. 2020) and major flowering-time loci such as Autoflower1 and Early1 (Toth et al. 2022).

For breeders the payoff is [marker-assisted selection](https://genetixbay.com/knowledge/glossary/marker-assisted-selection.md): screen seedlings for the markers and keep only those likely to carry the good [allele](https://genetixbay.com/knowledge/glossary/allele.md), before flowering.

## Don't confuse with

- **Gene**: a QTL is a region; the gene inside it may still be unknown.
- **Major gene**: a QTL with a large effect (like Autoflower1) behaves almost like a Mendelian gene; most QTL are small.
- **Proof of a trait**: a QTL shifts probabilities. Plants carrying the good allele still vary.

## Related terms

[Quantitative trait](https://genetixbay.com/knowledge/glossary/quantitative-trait.md), [Locus](https://genetixbay.com/knowledge/glossary/locus.md), [Marker-assisted selection](https://genetixbay.com/knowledge/glossary/marker-assisted-selection.md), [SNP](https://genetixbay.com/knowledge/glossary/snp.md), [Linkage](https://genetixbay.com/knowledge/glossary/linkage.md)

## Read more

- [Quantitative traits and heritability: why most good traits are hard to fix](https://genetixbay.com/knowledge/breeding/quantitative-traits-heritability.md)
- [Genotyping: what a DNA test can and can't prove](https://genetixbay.com/knowledge/breeding/genotyping-what-tests-prove.md)

## Sources

1. Petit J, Salentijn EMJ, Paulo MJ, Denneboom C, Trindade LM. 2020. Genetic architecture of flowering time and sex determination in hemp (Cannabis sativa L.): a genome-wide association study. Frontiers in Plant Science 11:569958. https://doi.org/10.3389/fpls.2020.569958
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
3. Barcaccia G, Palumbo F, Scariolo F, Vannozzi A, Borin M, Bona S. 2020. Potentials and challenges of genomics for breeding cannabis cultivars. Frontiers in Plant Science 11:573299. https://doi.org/10.3389/fpls.2020.573299