# Codominant

> Codominance is when both alleles at a locus show in a heterozygous plant, as with THCA and CBDA synthase, so a B_T/B_D plant makes both THC and CBD.

- Canonical: https://genetixbay.com/knowledge/glossary/codominant.html
- Also written: codominance, co-dominant, co-dominance, incomplete dominance
- 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"

Two [alleles](https://genetixbay.com/knowledge/glossary/allele.md) are codominant when a heterozygous plant expresses both of them rather than one masking the other. Incomplete dominance is a close relative: the heterozygote is intermediate between the two [homozygotes](https://genetixbay.com/knowledge/glossary/homozygous.md). Growers usually lump the two together.

## In practice

The cannabinoid synthase locus is the standard cannabis case. In de Meijer's model, B_T codes for [THCA synthase](https://genetixbay.com/knowledge/glossary/thca-synthase.md) and B_D for [CBDA synthase](https://genetixbay.com/knowledge/glossary/cbda-synthase.md). B_T/B_T plants are THC-dominant ([chemotype I](https://genetixbay.com/knowledge/glossary/chemotype.md)), B_D/B_D plants CBD-dominant (chemotype III), and B_T/B_D plants make both, typically in a ratio near 1:1 (chemotype II) (de Meijer et al. 2003).

This has a direct breeding consequence. Cross two balanced 1:1 plants and the offspring segregate about 1 THC-dominant : 2 balanced : 1 CBD-dominant. A "1:1 seed line" cannot be fixed by selection alone, because the balanced type is the heterozygote. The stable route is to cross a THC-dominant and a CBD-dominant parent, which gives uniform balanced [F1s](https://genetixbay.com/knowledge/glossary/f1.md).

## Units and formula

Expected offspring of B_T/B_D × B_T/B_D: 25% B_T/B_T, 50% B_T/B_D, 25% B_D/B_D. Real ratios vary because the region also contains duplicated and non-functional synthase copies (Laverty et al. 2019).

## Don't confuse with

- **Dominant**: one allele masks the other; in codominance neither is hidden.
- **Ratio inheritance**: the exact [THC:CBD ratio](https://genetixbay.com/knowledge/glossary/chemotype-ratio.md) within balanced plants still varies with other genes and environment.

## Related terms

[Dominant](https://genetixbay.com/knowledge/glossary/dominant.md), [Recessive](https://genetixbay.com/knowledge/glossary/recessive.md), [Allele](https://genetixbay.com/knowledge/glossary/allele.md), [Chemotype](https://genetixbay.com/knowledge/glossary/chemotype.md), [Heterozygous](https://genetixbay.com/knowledge/glossary/heterozygous.md)

## Read more

- [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