# What is the difference between THC and CBD plants?

> THC and CBD plants are the same species; the difference is which enzyme turns the shared precursor CBGA into cannabinoid acids. THC-dominant plants carry working THCA synthase, CBD-dominant plants CBDA synthase, and mixed 1:1 plants one of each. The trait is inherited as one near-codominant block, so THC x CBD gives mixed F1s and a 1:2:1 split in the F2.

- Canonical: https://genetixbay.com/knowledge/answers/thc-vs-cbd-plants.html
- Topic: Basics · Type: Definition
- Evidence: Established · Status: Draft: awaiting expert review
- Full article: [Chemotype inheritance: how THC and CBD synthase alleles pass on](https://genetixbay.com/knowledge/breeding/chemotype-inheritance.md)
- Also asked as: THC vs CBD plant | CBD strain vs THC strain | Hemp CBD vs THC cannabis | How is the THC to CBD ratio inherited? | Can a 1:1 THC:CBD plant breed true?
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Short answer

[THC](https://genetixbay.com/knowledge/glossary/thc.md) and [CBD](https://genetixbay.com/knowledge/glossary/cbd.md) plants are the same species with a different enzyme at one fork in the pathway. A THC-dominant plant turns the precursor [CBGA](https://genetixbay.com/knowledge/glossary/cbga.md) into [THCA](https://genetixbay.com/knowledge/glossary/thca.md); a CBD-dominant plant turns it into [CBDA](https://genetixbay.com/knowledge/glossary/cbda.md); a mixed plant does both. The trait inherits as one near-codominant block. *Established* genetics.

## Why

One precursor, competing enzymes. CBGA sits at a fork, and [THCA synthase](https://genetixbay.com/knowledge/glossary/thca-synthase.md) and [CBDA synthase](https://genetixbay.com/knowledge/glossary/cbda-synthase.md) compete for it. Crossing pure THC and pure CBD plants gave uniformly mixed [F1s](https://genetixbay.com/knowledge/glossary/f1.md) and a 1:2:1 split in the [F2](https://genetixbay.com/knowledge/glossary/f2.md), explained by one locus with [codominant](https://genetixbay.com/knowledge/glossary/codominant.md) [alleles](https://genetixbay.com/knowledge/glossary/allele.md) [1].

The "locus" is really a [gene](https://genetixbay.com/knowledge/glossary/gene.md) block. The two synthase genes sit in large, repeat-rich, low-recombination regions, so they pass on together [2]. CBD in drug-type lines often traces to [hemp](https://genetixbay.com/knowledge/glossary/hemp.md) bred into marijuana backgrounds [3].

[Seedlings](https://genetixbay.com/knowledge/glossary/seedling.md) can be screened. Validated DNA markers identify [chemotype](https://genetixbay.com/knowledge/glossary/chemotype.md) from young plants [4], weeks before flower.

How much cannabinoid a plant makes is a separate, many-gene trait. Chemotype sets the ratio, not the potency.

## The numbers

| Chemotype | Ratio | Synthase alleles | Breeds true? |
|---|---|---|---|
| I, THC-dominant | THC:CBD > ~10:1 | THC / THC | Yes |
| II, mixed | ~0.5:1 to 3:1 | THC / CBD | No: 1:2:1 |
| III, CBD-dominant | CBD:THC > ~10:1 (hemp often 20–30:1) | CBD / CBD | Yes |
| IV, CBG-dominant | High [CBG](https://genetixbay.com/knowledge/glossary/cbg.md) | Non-functional, both copies | Yes, once homozygous |

| Cross | Offspring |
|---|---|
| I x III | 100% II |
| II x II | 25% I : 50% II : 25% III |
| II x III | 50% II : 50% III |

## What to do

1. Want a 1:1 plant? Keep it as a clone; its seed won't hold the ratio.
2. Breeding CBD into a THC line? Cross, then select type III in the F2 or backcross.
3. Screen seedlings with a chemotype test, then confirm with flower tests.
4. Growing CBD as hemp? Test before harvest; rules vary, so check local limits.

## Common mistakes

- Expecting a 1:1 cultivar to breed true.
- Assuming CBD plants contain no THC.
- Confusing chemotype (the ratio) with potency (the amount).

## Related

- Full article: [Chemotype inheritance](https://genetixbay.com/knowledge/breeding/chemotype-inheritance.md)
- Answers: [what THCA is](https://genetixbay.com/knowledge/answers/what-is-thca.md) · [hemp vs marijuana](https://genetixbay.com/knowledge/answers/hemp-vs-marijuana.md)
- Article: [Mendelian genetics for growers](https://genetixbay.com/knowledge/breeding/mendelian-genetics-for-growers.md)
- Terms: [chemotype](https://genetixbay.com/knowledge/glossary/chemotype.md) · [THCA synthase](https://genetixbay.com/knowledge/glossary/thca-synthase.md) · [CBDA synthase](https://genetixbay.com/knowledge/glossary/cbda-synthase.md)

## Follow-up questions

**How is the THC to CBD ratio inherited?** The THC to CBD ratio in cannabis is inherited as one block, close to codominant. The THCA synthase and CBDA synthase genes are separate but tightly linked in low-recombination regions, so they pass on together like a single locus with two alleles. Two copies of either give a pure type; one of each gives a mixed type II plant.

**Can a 1:1 THC:CBD plant breed true?** No, a 1:1 THC:CBD cannabis plant cannot breed true. Mixed chemotype II plants carry one THC synthase and one CBD synthase allele, so their seed segregates about 25% THC-dominant, 50% mixed and 25% CBD-dominant. Keep a 1:1 cultivar as a clone, or remake it each time by crossing a pure THC parent with a pure CBD parent.

**Do CBD plants contain any THC?** Yes, CBD-dominant cannabis plants still contain some THC. Even good CBD hemp lines typically show THC:CBD ratios around 1:20 to 1:30, so a plant at 15% CBD can carry about 0.5–0.75% THC. That is why CBD crops grown as hemp can test over a 0.3% legal limit. Test before harvest and check your local rules.

## 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. Grassa CJ, Weiblen GD, Wenger JP, et al. 2021. A new Cannabis genome assembly associates elevated cannabidiol (CBD) with hemp introgressed into marijuana. New Phytologist 230(4):1665-1679. https://doi.org/10.1111/nph.17243
4. Toth JA, Stack GM, Cala AR, et al. 2020. Development and validation of genetic markers for sex and cannabinoid chemotype in Cannabis sativa L. GCB Bioenergy 12(3):213-222. https://doi.org/10.1111/gcbb.12667