Last updated 26 October 2026 · Originally published 4 February 2021
A cannabis plant’s THC to CBD ratio is determined mainly by genetics, not by light, nutrients, or growing technique. The genes it inherits decide which cannabinoid dominates; growing conditions mostly change how much cannabinoid it produces, not which one wins.
One starting molecule, two possible endings
Cannabis makes THC and CBD through the same chemical pathway. It first produces cannabigerolic acid, or CBGA, sometimes called the precursor to the major cannabinoids. We cover it separately in our CBGA article.
From CBGA, the pathway splits:
- THCA synthase converts CBGA into THCA, the acid form of THC.
- CBDA synthase converts CBGA into CBDA, the acid form of CBD.
The two enzymes compete for the same starting material. Which enzymes work — and how well — depends largely on the cannabinoid-synthase genes the plant inherited.
Early breeding experiments made this look like one gene with two versions. Modern genome sequencing shows a more complicated cluster containing several synthase genes and inactive copies. Because these genes sit close together and are usually inherited as a unit, the simpler inheritance model still predicts whether a plant will be THC-dominant, CBD-dominant, or intermediate.
Neither THCA nor CBDA produces the familiar THC high, although both are biologically active. Heat and, more slowly, storage remove carbon dioxide from the molecules, converting THCA into THC and CBDA into CBD. This process is called decarboxylation.
The three main chemotypes
Researchers generally divide cannabis into three chemical types — a classification our piece on Ernest Small and the three cannabis chemotypes covers in more depth:
- Chemotype I: THC-dominant. THC greatly exceeds CBD.
- Chemotype II: intermediate. The plant produces substantial amounts of both, often close to a 1:1 ratio.
- Chemotype III: CBD-dominant. CBD greatly exceeds THC.
Many fibre, grain, and CBD-hemp cultivars are chemotype III. But hemp is a legal and agricultural category, not a chemotype: a CBD-dominant plant can still produce enough THC to exceed the legal limit.
Two less common chemotypes are also recognized. Chemotype IV plants are dominated by CBG, while chemotype V plants produce few or no detectable cannabinoids. For the ordinary THC-versus-CBD question, though, the first three explain most plants.
Genetics sets the ratio, not the amount
A 2020 study in the American Journal of Botany tested this inheritance model in more than 300 feral cannabis plants from Minnesota, 20 clinical cultivars, and 12 Canadian grain cultivars. The plants’ synthase genotypes accurately predicted whether they were THC-dominant, CBD-dominant, or intermediate.
What the genes did not predict nearly as well was total potency. Plants with the same basic chemotype could still produce very different total amounts of cannabinoids.
That’s where genetics gets more complicated — and where growing conditions matter. Other genes influence flower structure, trichome production, and the earlier stages of cannabinoid synthesis. Light, nutrition, maturity, and environmental stress can all affect final cannabinoid content.
Put simply: genetics largely determines the ratio, while genetics and growing conditions together determine the amount.
Why the strain name is not enough
A strain name is not a reliable cannabinoid profile. A genetic study of cannabis sold under 30 popular names found considerable inconsistency between samples carrying the same name, though some became reasonably consistent once obvious outliers were removed. The researchers also found isolated cases in which products with different names appeared genetically identical. Appearance doesn’t settle it either — see our guide to identifying cultivars by appearance for why.
The useful number is the measured THC:CBD ratio for that particular batch. That comes from its certificate of analysis, or COA, not from the strain name.
A COA usually reports both the acidic and neutral cannabinoids. “Total CBD” is calculated as:
CBD + (CBDA × 0.877)
The adjustment accounts for the weight lost when CBDA releases carbon dioxide and becomes CBD. Total THC is calculated the same way from THC and THCA. Our COA guide explains how to read the full report, and our spectrums guide covers how this ratio plays out in full-spectrum, broad-spectrum, and isolate products.
Why genetics matters legally
Genetics also affects the risk that a hemp crop will test “hot” — above the legal THC limit. Licensed US hemp production already uses testing that counts the potential THC available from THCA.
Under federal changes scheduled for 11 December 2026, the legal definition of hemp will expressly limit plants to 0.3% total THC, including THCA, on a dry-weight basis. A separate rule will cap finished hemp-derived cannabinoid products at 0.4 mg of combined total THC and similarly acting cannabinoids per container.
These rules limit measured THC, not the activity of a particular gene. But plants with more active THC-producing genetics naturally face a greater risk of exceeding the limit. Our hemp-law article covers the new rules in full.
Does the ratio tell you how it will feel?
Genetics decides the ratio a plant produces. It doesn’t decide how that ratio feels once it’s consumed. Controlled trials haven’t shown that CBD reliably blunts THC’s effects — one 2023 randomized trial found that 640 mg of oral CBD, an unusually high dose, actually intensified 20 mg of oral THC’s effects rather than softening them. A CBD-rich product doesn’t guarantee a gentler experience; our article on whether CBD takes the edge off THC has the full picture.
The bottom line
A cannabis plant’s basic THC:CBD ratio is largely inherited. Growing conditions can change how much cannabinoid it produces, but they usually cannot turn a THC-dominant plant into a CBD-dominant one.
For consumers, the practical lesson is simple: read the batch’s lab results, not the strain name. The strain name tells a story; the COA tells you what the plant actually produced — and for what CBD itself does once it’s in the body, see our CBD learning hub.
