# Allele

> An allele is one version of a gene; a diploid cannabis plant carries two alleles at each locus, one from each parent, which may be the same or different.

- Canonical: https://genetixbay.com/knowledge/glossary/allele.html
- Also written: alleles, gene variant, allelic variant
- 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"

An allele is one version of a [gene](https://genetixbay.com/knowledge/glossary/gene.md). Cannabis is diploid, so every plant carries two alleles at each [locus](https://genetixbay.com/knowledge/glossary/locus.md), one inherited through the pollen and one through the ovule. If the two are the same the plant is homozygous at that locus; if they differ it is [heterozygous](https://genetixbay.com/knowledge/glossary/heterozygous.md).

## In practice

The best-known cannabis example is the cannabinoid synthase locus. One allele (often written B_T) makes a functional [THCA synthase](https://genetixbay.com/knowledge/glossary/thca-synthase.md); another (B_D) makes [CBDA synthase](https://genetixbay.com/knowledge/glossary/cbda-synthase.md). A plant carrying B_T/B_T is THC-dominant, B_D/B_D is CBD-dominant, and B_T/B_D produces both in a roughly balanced ratio (de Meijer et al. 2003). Breeders who want a stable 1:1 [THC](https://genetixbay.com/knowledge/glossary/thc.md):[CBD](https://genetixbay.com/knowledge/glossary/cbd.md) seed line learn quickly why it cannot breed true: the balanced plants are heterozygous and their offspring segregate.

Alleles are written with letters (A and a) for teaching. Real cannabis loci can have more than two alleles in the population, and some regions contain duplicated, non-functional copies of synthase genes (Laverty et al. 2019).

## Don't confuse with

- **Gene**: the position and function; the allele is the variant at that position.
- **Mutation**: the event that creates a new allele. Most alleles in a population are old, not new.
- **Phenotype**: what you see. Two different allele pairs can produce the same visible result when one is [dominant](https://genetixbay.com/knowledge/glossary/dominant.md).

## Related terms

[Gene](https://genetixbay.com/knowledge/glossary/gene.md), [Locus](https://genetixbay.com/knowledge/glossary/locus.md), [Homozygous](https://genetixbay.com/knowledge/glossary/homozygous.md), [Heterozygous](https://genetixbay.com/knowledge/glossary/heterozygous.md), [Dominant](https://genetixbay.com/knowledge/glossary/dominant.md), [Recessive](https://genetixbay.com/knowledge/glossary/recessive.md)

## Read more

- [Mendelian genetics for growers, with Punnett squares](https://genetixbay.com/knowledge/breeding/mendelian-genetics-for-growers.md)
- [Chemotype inheritance: how THC and CBD synthase alleles pass on](https://genetixbay.com/knowledge/breeding/chemotype-inheritance.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. Laverty KU, Stout JM, Sullivan MJ, et al. 2019. A physical and genetic map of Cannabis sativa identifies extensive rearrangements at the THC/CBD acid synthase loci. Genome Research 29(1):146-156. https://doi.org/10.1101/gr.242594.118
3. Acquaah G. 2012. Principles of Plant Genetics and Breeding, 2nd edition. Wiley-Blackwell. https://doi.org/10.1002/9781118313718