# Chemotype inheritance: how THC and CBD synthase alleles pass on

> Whether a plant is THC dominant, CBD dominant or mixed is controlled mainly by a single inheritance block carrying THCA synthase and CBDA synthase genes, which behaves close to codominant: THC x CBD gives mixed (type II) F1s and a 1:2:1 split in the F2. CBG-dominant and cannabinoid-free types come from non-functional synthase alleles. Total cannabinoid amount is a separate, quantitative trait.

- Canonical: https://genetixbay.com/knowledge/breeding/chemotype-inheritance.html
- Category: Breeding · Level: Breeder
- Evidence: Established · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Flower · Systems: Any system
- 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)"

## Questions and answers

**If I cross a [THC](https://genetixbay.com/knowledge/glossary/thc.md) plant with a CBD plant what do I get?** Crossing a THC-dominant cannabis plant with a CBD-dominant one gives mixed chemotype II [F1](https://genetixbay.com/knowledge/glossary/f1.md) offspring, roughly 1:1 to 1:2 THC:CBD, when both parents are [homozygous](https://genetixbay.com/knowledge/glossary/homozygous.md). Any pure THC or pure CBD plant in the F1 means a parent wasn't homozygous. Intercrossing those mixed F1s gives about 1 THC : 2 mixed : 1 CBD in the [F2](https://genetixbay.com/knowledge/glossary/f2.md).

**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. Research models this as B_T and B_D: two copies of either give a pure type, one of each gives mixed.

**What are chemotypes I, II, III, IV and V?** Cannabis chemotypes are classes by [cannabinoid](https://genetixbay.com/knowledge/glossary/cannabinoid.md) profile. Type I is THC-dominant (THC:CBD above about 10:1), type II is mixed (about 0.5:1 to 3:1), type III is CBD-dominant (CBD:THC above about 10:1), type IV is CBG-dominant from non-functional synthase alleles, and type V has only trace cannabinoids because the pathway is blocked upstream. Boundaries vary between studies.

**Can a 1:1 THC:CBD plant breed true?** No, a 1:1 THC:CBD cannabis plant cannot breed true. Mixed chemotype II plants are [heterozygous](https://genetixbay.com/knowledge/glossary/heterozygous.md), carrying one THC synthase and one CBD synthase allele, so their seed [segregates](https://genetixbay.com/knowledge/glossary/segregation.md) 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 pure THC and pure CBD parents.

**How are CBG-dominant plants made?** CBG-dominant cannabis (chemotype IV) is made by breeding plants homozygous for non-functional [THCA](https://genetixbay.com/knowledge/glossary/thca.md) and CBDA synthase alleles. With neither enzyme working, the precursor CBGA builds up instead of being converted. Research shows these null alleles inherit predictably, so once a line is homozygous for them it [breeds true](https://genetixbay.com/knowledge/glossary/true-breeding.md) for the CBG-dominant chemotype.

**Can I predict THC percentage from genetics?** Genetics can predict a cannabis plant's chemotype class but not its THC percentage. Which synthase a plant carries sets the THC:CBD ratio, while total cannabinoid amount is a separate many-gene quantitative trait that is sensitive to environment and lab. Seedling DNA or leaf tests predict class well; select for potency only from repeated flower tests at the same stage and lab.

**Short answer.** CBGA is the shared precursor; a THCA synthase turns it into THCA and a CBDA synthase into CBDA. Which functional synthase a plant carries is inherited as one block, close to codominant. THC-dominant x CBD-dominant gives mixed F1s (roughly 1:1 to 1:2 THC:CBD); intercross those and you get about 1 THC : 2 mixed : 1 CBD. Mixed plants never breed true. How *much* total cannabinoid a plant makes is a separate, many-gene trait.

## What's happening

- **The pathway.** CBGA sits at a fork. THCAS, CBDAS and CBCAS compete for it. If neither THCAS nor CBDAS works, CBGA builds up (chemotype IV) [2]. If CBGA itself is barely made (or trichome development fails), the plant is near cannabinoid-free (chemotype V) [8].
- **The 2003 model.** de Meijer et al. crossed inbred pure-THC and pure-CBD plants and explained the results with one locus, B, with codominant alleles B_T and B_D [1]. The F1 was uniformly mixed; F2s segregated 1:2:1.
- **The genomic picture.** THCAS and CBDAS are separate but closely [linked genes](https://genetixbay.com/knowledge/glossary/linkage.md), sitting in large (>250 kb), retrotransposon-rich, low-recombination regions that differ structurally between drug and hemp types [3][4]. In practice they still inherit together, so the one-locus model predicts well, but "B_T" and "B_D" are really haplotypes (gene blocks), not single genes. Non-functional copies and gene duplications exist [3].
- **CBD in drug-type lines** often traces to hemp [introgressed](https://genetixbay.com/knowledge/glossary/introgression.md) into marijuana backgrounds [5] (*Supported*).

## Chemotypes

| Chemotype | Typical ratio | Inferred synthase genotype | Breeds true? |
|---|---|---|---|
| I, THC dominant | THC:CBD > ~10:1 (often far higher) | B_T / B_T | Yes, for chemotype |
| II, mixed | ~0.5:1 to 3:1 | B_T / B_D | No: segregates 1:2:1 |
| III, CBD dominant | CBD:THC > ~10:1 (hemp lines often 20-30:1) | B_D / B_D | Yes, for chemotype |
| IV, CBG dominant | High CBG, low THC and CBD | Null (non-functional) synthase alleles, homozygous [2] | Yes, once homozygous |
| V, near zero | Trace cannabinoids | Pathway blocked upstream [8] | Yes |

Ratio boundaries are approximate and vary between studies [1][6]. Note: "CBD dominant" plants still make some THC; even good hemp lines show THC:CBD ratios around 1:20 to 1:30 because CBDAS itself makes a little THCA.

## Punnett square: THC x CBD

F1 (B_T/B_D) x F1 (B_T/B_D):

| | B_T | B_D |
|---|---|---|
| **B_T** | I (THC) | II (mixed) |
| **B_D** | II (mixed) | III (CBD) |

Backcross F1 (B_T/B_D) to the CBD parent (B_D/B_D): 1 mixed : 1 CBD. Two backcrosses with selection for type III recover a CBD-dominant line with the THC parent's other traits partly kept.

## Step by step: breeding a stable CBD-dominant line

1. Cross your THC-dominant selection (female) with a stable CBD-dominant male.
2. Grow F1s; all should be type II. Any type I or III F1 means a parent wasn't homozygous.
3. Choose the best F1 (or backcross to the CBD parent).
4. In the F2 or BX1, **screen seedlings with a chemotype DNA marker** if available [7], or test a leaf or flower sample by chromatography.
5. Keep only type III plants; select among them for the traits you want.
6. Confirm the ratio with flower tests (same stage, same lab) on several plants of the final line.

## Numbers that matter

| Population | Expected chemotype split |
|---|---|
| I x III (both homozygous) | 100% II |
| II x II | 25% I : 50% II : 25% III |
| II x III | 50% II : 50% III |
| II x I | 50% I : 50% II |
| Plants needed for 95% chance of ≥1 type III in an F2 | 11 (p = 0.25) |
| ...of ≥5 type III in an F2 | 34 (binomial, p = 0.25) |

## Common mistakes

- Expecting a 1:1 cultivar to breed true. It is heterozygous by definition.
- Selecting "high THC" from small, single-run lab results. [Total THC](https://genetixbay.com/knowledge/glossary/total-thc.md) is quantitative and environment- and lab-sensitive.
- Assuming a seedling leaf test predicts the final ratio exactly. It predicts chemotype class well; ratios shift through flowering (*Supported*).
- Forgetting legal limits: in hemp jurisdictions, total THC limits apply to the crop, and type II segregants can exceed them.

## Evidence

- Single inheritance unit, codominant model: *Established* [1], refined by genomics [3][4].
- CBG-dominant inheritance via null alleles: *Established* [2].
- Chemotype markers for seedling screening: *Supported* [7]; marker performance may vary by germplasm.
- Hemp introgression as a CBD source in drug-type lines: *Supported* [5].

## 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. de Meijer EPM, Hammond KM. 2005. The inheritance of chemical phenotype in Cannabis sativa L. (II): Cannabigerol predominant plants. Euphytica 145:189-198. https://doi.org/10.1007/s10681-005-1164-8
3. Weiblen GD, Wenger JP, Craft KJ, et al. 2015. Gene duplication and divergence affecting drug content in Cannabis sativa. New Phytologist 208(4):1241-1250. https://doi.org/10.1111/nph.13562
4. 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
5. 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
6. Hillig KW, Mahlberg PG. 2004. A chemotaxonomic analysis of cannabinoid variation in Cannabis (Cannabaceae). American Journal of Botany 91(6):966-975. https://doi.org/10.3732/ajb.91.6.966
7. 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
8. 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