How THC And CBD Ratios Are Influenced By Genetics

Why do strains like Blue Dream and Harlequin have such different effects? In large part, it’s because they have very different THC-to-CBD ratios.

THC and CBD are the two most abundant cannabinoids in most strains. THC is well known as the major psychoactive compound. CBD is best known for having a wide range of medical uses. While CBD lacks the psychoactive properties of THC, it does influence the effects of THC in the brain. This is why the THC:CBD ratio strongly influences a strain’s effects, and why that ratio is important when deciding which strain is right for you.

Here’s the cool part: The THC:CBD ratio is largely determined by strain genetics. Each plant’s genetic code determines the way the plant produces the two compounds. It’s a fascinating process that many consumers aren’t aware of. 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.

There Are Three Cannabis 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.

Cannabis genetics limit THC and CBD production so that only these three broad categories of flower are possible. Hemp strains do not produce significant levels of THC, while most commercial strains fall into the THC-dominant category—they have THC but negligible levels of CBD. “Balanced strains” produce both THC and CBD, but generally not as much THC as THC-dominant strains or as much CBD as the more potent hemp strains.

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.

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