A chemotype is the chemical phenotype of a plant. In cannabis research it normally means the cannabinoid class, set largely by the alleles at the cannabinoid synthase locus. The usual scheme: chemotype I is THC-dominant, II is mixed THC and CBD, III is CBD-dominant, IV is CBG-dominant, and V has almost no cannabinoids (de Meijer et al. 2003; de Meijer et al. 2009).
In practice
Chemotype is the first thing to fix in a medicinal or hemp line and the easiest to test early. Breeders can screen seedlings with a DNA marker for the synthase alleles (Toth et al. 2020) and keep only the wanted chemotype before flowering. A THC-dominant × CBD-dominant cross gives uniform type II F1s; their F2s segregate back into I, II and III.
Some writers extend "chemotype" to terpene profile ("a limonene chemotype"). That use is looser and less standardised; terpene profiles are controlled by many genes and shift with growing conditions.
Don't confuse with
- Chemovar: a named cultivar defined by its chemistry. A chemotype is a class; a chemovar is a variety.
- Potency: chemotype is the type of cannabinoid, not how much. Two type I plants can test 12% or 25%.
- Indica or sativa: these labels do not predict chemotype (Smith et al. 2022).
A cannabinoid is any compound that binds the body's cannabinoid receptors or shares the typical cannabis terpenophenolic structure; the plant-made ones are phytocannabinoids such as THCA and CBDA.An allele is one version of a gene; a diploid cannabis plant carries two alleles at each locus, one from each parent, which may be the same or different.THC (Δ9-tetrahydrocannabinol) is the neutral, decarboxylated cannabinoid mainly responsible for cannabis's reported intoxicating effects; the living plant makes it almost entirely as THCA.CBD (cannabidiol) is a non-intoxicating neutral cannabinoid formed when CBDA decarboxylates; hemp-type and CBD-dominant cannabis make it mainly as CBDA in their trichomes.F1 is the first generation of offspring from crossing two parents; if the parents are inbred and different, F1 plants are uniform and often show hybrid vigour.F2 is the second filial generation, made by crossing or selfing F1 plants; it shows the widest segregation, bringing back hidden recessives and new combinations.A terpene profile is the measured mix of terpenes in a sample, reported as each compound's % w/w and the total, used to describe and compare cultivar aroma chemistry.A chemovar is a cultivar defined and named by its chemical profile, chiefly cannabinoids and terpenes, rather than by its appearance or a market strain name.Codominance is when both alleles at a locus show in a heterozygous plant, as with THCA and CBDA synthase, so a B_T/B_D plant makes both THC and CBD.The phenotype is what a plant visibly and measurably is (shape, colour, smell, chemistry, timing), the result of its genotype acting in a particular environment.Genotyping is reading a plant's DNA at selected markers to identify it, compare it with other plants, check sex or chemotype, or estimate its ancestry.
