Cannabis is an extremely interesting plant which has the ability to make hundreds of compounds which are gaining worldwide attention for therapeutic potential. The relaxation of tight regulations has brought a thriving industry and interest from consumers in Cannabis products. Cannabinoids are almost exclusive to Cannabis plants, although there are a handful of other plants that produce cannabinoids in low quantities. Although there are at least 120 cannbinoids that have been isolated and described, there are generally three that are considered the major cannbinoids.
Most of us have heard about THC (tetrahydrocannabinol) and CBD (cannabidiol), and perhaps CBG (cannabigerol). Interestingly, the plant doesn't actually make these compounds, but rather makes the acidic compounds THCA (tetrahydrocannabinolic acid), CBDA (cannabidiolic acid) and CBGA (cannabigerolic acid), which have a carboxyl group on the third ring of the molecule. This carboxyl is removed through a decarboxylation process, usually achieved through heating the plant material.
Cannabigerolic acid (CBGA) is the precursor molecule that is made into THCA and CBDA through enzymatic action.
https://doi.org/10.1111/gcbb.12667 Toth et al. 2020
Gene combinations and enzymatic activity in the plant determine the cannabinoid content in the final phenotype (the physical observable characteristics of an organism).
A specific region in the genome called the BT allele contains the tetrahydrocannabinolic acid synthase (THCAS) gene that codes for the enzyme responsible for converting CBGA to THCA. Conversely, the BD allele contains the cannabidiolic acid synthase (CBDAS) gene that codes for the enzyme responsible for converting CBGA to CBDA.
These genes appear to follow a codominant Mendelian mode of inheritance; two copies of the BT allele (BT BT) results in a THC dominant plant, two copies of the BD allele (BD BD) results in a CBD dominant plant, and one copy of each allele (BT BD) results in an intermediate type plant with moderate levels of both THC and CBD production.
https://doi.org/10.1111/gcbb.12667 Toth et al. 2020
The above diagram depicts data showing Genotype to chemotype relationships. (a) Total potential tetrahydrocannabinol (THC) and cannabidiol (CBD) concentration (% dry mass) in individual plants for which B locus genotype was also determined. The red line indicates 0.3% total potential THC (b) Δ9‐THC concentration by genotype. The red line indicates 0.3% dry weight Δ9‐THC. (c) Total potential THC concentration by genotype. The red line indicates 0.3% dry weight total potential THC. (d) Total potential CBD:THC concentration ratio.
These gene combinations and synthase action have the potential to result in five categories of Cannabis. Describing a Cannabis plant based on this system is more informative than the current hemp and marijuana descriptions that are based solely on THC content below or above 0.3% and holds no biological significance.
Type I is THC dominant (BT BT) generally >10% THC with low levels of CBD (<0.5% dry weight).
Type II has a mixed CBD and THC profile (BT BD) with moderate THC and CBD with an approximate 1:1 ratio.
Type III is CBD dominant (BD BD), generally >10% CBD with low levels of THC.
Type IV and V result due to low-functioning or non-functioning genes in the biosynthetic pathways leading to THC and CBD production. Type IV is CBG dominant, with low levels of THC and CBD, possibly due to low or non-functional synthase genes, thereby unable to convert CBGA to THCA and/or CBDA. Type V produces little to no cannabinoids, indicating a problem with genes upstream in the biosynthetic pathway for CBGA.
The mechanism whereby (A) Type IV express CBG dominant chemotype due to low or non-functional THCA and CBDA synthase genes and are unable to convert CBGA to THCA and/or CBDA possibly due to low or non-functional synthase genes, thereby unable to convert CBGA to THCA and/or CBDA, and (B) Type V produces little to no cannabinoids, indicating a problem with genes upstream in the biosynthetic pathway for CBGA (Toth et al. 2020).
A recent study assessed cannabinoid levels in feral, clinical, and industrial Cannabis plants and found Type I has much higher ratio of THC:CBD, Type II has a relatively balanced ratio of THC:CBD and Type III has a much lower ratio of THC:CBD. These phenotypes were predicted using genetic information, which could be a useful tool to use in the industry. Genetic data to predict or determine the type of plant being grown could help the industry, guide policy makers, and assist law enforcement when trying to establish the purpose of plants being grown.
10.1002/ajb2.1550 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7702092/ Wenger et al. 2020
I like this categorization scheme, as it gives a good idea of the cannabinoid ratios which is more important than the magical mystery 0.3% THC threshold currently used to categorize Cannabis plants.