# Epistasis

> Epistasis is an interaction between genes in which the effect of one gene depends on, or is masked by, alleles at another gene.

- Canonical: https://genetixbay.com/knowledge/glossary/epistasis.html
- Also written: epistatic, epistatic interaction, gene interaction, gene-gene interaction
- 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"

Epistasis is when the effect of one [gene](https://genetixbay.com/knowledge/glossary/gene.md) depends on which [alleles](https://genetixbay.com/knowledge/glossary/allele.md) are present at another gene. One gene can mask another, enhance it, or only matter in combination with it. [Dominance](https://genetixbay.com/knowledge/glossary/dominant.md) is an interaction between alleles at one locus; epistasis is between loci.

## In practice

Pathway genes give the cleanest examples. A [cannabinoid](https://genetixbay.com/knowledge/glossary/cannabinoid.md) synthase allele cannot do anything if the plant lacks enough of the precursor CBGA upstream; that is part of how [chemotype](https://genetixbay.com/knowledge/glossary/chemotype.md) IV (CBG-dominant) and chemotype V (cannabinoid-free) plants arise (de Meijer et al. 2009). The synthase genotype only shows when the upstream genes work.

For breeders, epistasis explains why a great combination sometimes vanishes in the next generation: the good result depended on alleles at two or more genes being together, and [segregation](https://genetixbay.com/knowledge/glossary/segregation.md) split them. It is also one reason an [elite clone](https://genetixbay.com/knowledge/glossary/elite-cut.md) can be easier to keep than to recreate from seed.

## Units and formula

Classic modified F2 ratios signal epistasis: 9:3:4 ([recessive](https://genetixbay.com/knowledge/glossary/recessive.md) epistasis), 12:3:1 (dominant epistasis), 9:7 (complementary genes), instead of the 9:3:3:1 of two independent genes.

## Don't confuse with

- **Dominance**: within one gene.
- **Pleiotropy**: one gene affecting several traits.
- **Linkage**: two genes inherited together because of their position, not because they interact.

## Related terms

[Dominant](https://genetixbay.com/knowledge/glossary/dominant.md), [Quantitative trait](https://genetixbay.com/knowledge/glossary/quantitative-trait.md), [Heterosis](https://genetixbay.com/knowledge/glossary/heterosis.md), [Gene](https://genetixbay.com/knowledge/glossary/gene.md), [Pleiotropy](https://genetixbay.com/knowledge/glossary/pleiotropy.md)

## Read more

- [Quantitative traits and heritability: why most good traits are hard to fix](https://genetixbay.com/knowledge/breeding/quantitative-traits-heritability.md)

## Sources

1. de Meijer EPM, Hammond KM, Sutton A. 2009. The inheritance of chemical phenotype in Cannabis sativa L. (IV): cannabinoid-free plants. Euphytica 168:95-112. https://doi.org/10.1007/s10681-009-9894-7
2. Falconer DS, Mackay TFC. 1996. Introduction to Quantitative Genetics, 4th edition. Longman, Harlow. ISBN 978-0-582-24302-6