Chemotype: the chemical identity of a cannabis strain

In short

The chemotype describes which cannabinoids a plant makes and in what ratio. It says more about a strain than the name on the packet, because it rests on measurable lab values rather than marketing.

The three basic types

The classification goes back to Ernest Small and H. D. Beckstead, who in 1973 distinguished three chemical types by the ratio of THC to CBD. That framework still holds:

Type I is THC-dominant, with a THC-to-CBD ratio above one. This is the classic intoxicating type.
Type II is balanced, with THC and CBD roughly level.
Type III is CBD-dominant with very little THC. Industrial hemp falls here.

Note that chemotype means the ratio only, not the absolute amount. A plant with 10 % and one with 25 % THC can both be Type I, as long as THC is the dominant cannabinoid.

The rarer types IV and V

Two more types were added later. Type IV makes mainly cannabigerol (CBG) instead of THC or CBD; Fournier described it in 1987. Type V was proposed in 2004 by Mandolino and Carboni for plants with virtually no detectable cannabinoids. Both are rare and matter mainly in breeding and research.

Where the chemotype comes from

Whether a plant becomes Type I, II or III is decided at the B-Locus. The codominant variants for THC and CBD fix which of the two cannabinoids dominates. That is why chemotype can be bred on purpose: cross a pure THC type with a pure CBD type and the first generation is Type II.

Why chemotype rather than indica or sativa

For effect and for breeding work, chemotype is the sturdier category. Strain names and the labels indica or sativa say little about a plant’s actual chemistry, while the THC-CBD ratio is clearly measurable in the lab. For a full description, though, the terpene profile belongs alongside the chemotype.