Every cannabis consumer knows that different strains produce different effects. And while some of this can be attributed to varying levels of terpenes, differences in effects are largely due to variations in the THC to CBD ratio.

But the ratio of THC to CBD doesn’t exist on a continuous spectrum. Instead, it tends to fall into one of three distinct categories or chemotypes.

A chemotype is a subgroup within a species of plant or microorganism. Chemotypes have distinct chemical compositions due to differences in the production of secondary metabolites—cannabinoids and terpenes in the case of the cannabis plant. These differences are the result of relatively minor genetic changes that have little effect on the anatomy of the plant but produce unique chemical characteristics.

The three chemotypes of cannabis are:

  • Chemotype I: THC dominant with high levels of THC and low levels of CBD
  • Chemotype II: Balanced with moderate levels of both THC and CBD
  • Chemotype III: CBD dominant with high levels of CBD and low levels of THC

Although every strain is unique, knowing the general chemotype will help a consumer understand what kind of effect they can expect from a strain. For example, CBD dominant (chemotype III) strains will not be as psychoactive as THC dominant (chemotype I) strains.

Strain Genetics Determine the Chemotype

So what determines a stain’s chemotype?

THC and CBD production occurs via complex biosynthetic pathways in the cannabis plant. Cannabigerol A (CBGA) is the main precursor cannabinoid for both CBD and THC.

From CBGA, the enzymes CBDA synthase or THCA synthase produce CBDA and THCA respectively, which are then converted into CBD and THC. Therefore, the regulation of these two enzymes plays a key role in determining the chemotype of a given cannabis strain.

In 2003, a group of researchers from the Netherlands demonstrated that a strain’s chemotype is controlled by genetic inheritance. The researchers also speculated that the chemotype was controlled by a single gene with two possible mutational variants or alleles.

Although this inheritance model still holds true in the practice of cannabis breeding, more recent research suggests that multiple, tightly linked cannabinoid synthase genes are responsible for determining cannabis chemotype.

The genes that encode for CBDA synthase and THCA synthase are two obvious candidates. And so far, the research confirms their importance in determining chemotype. For example, one 2017 study showed that low THC strains carried a gene encoding for an inactive form of THCA synthase while high THC strains carried a gene encoding for an active form of THCA synthase.


Learning more about the pathways that produce THC and CBD in cannabis can help growers enact more informed breeding programs. And with researchers now focusing on the structure and function of CBDA synthase and THCA synthase, we may see lab-produced cannabinoids hitting the shelves in the future.