Cannabis Strain Genetics Explained

Short answer

A cannabis strain is a cultivated variety. Its traits come from DNA. Sellers group strains as sativa, indica, or hybrid. Those three words describe plant shape, flowering time, and the region a line came from. They do not predict effect on a person. Lab tests of THC, CBD, and terpenes give data that labels do not.

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The plant and its genome

Cannabis sativa L. is one species. Plants in the species range from fiber hemp to high-THC drug types. Published assemblies place the haploid genome near 800 to 850 million base pairs. The plant has nine autosome pairs plus a sex chromosome pair. Male plants carry XY. Female plants carry XX. The gene that builds tetrahydrocannabinolic acid, THCA, is THCAS. A related gene, CBDA synthase, builds cannabidiolic acid. Which version of those genes a plant carries, and how active they are, sets the THC to CBD ratio.

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Sativa, indica, ruderalis

• Sativa: tall plants, narrow leaflets, long flowering period. Lines came from equatorial regions.
• Indica: shorter plants, wider leaflets, shorter flowering period. Lines came from Central and South Asia.
• Ruderalis: short plants, autoflowering. Lines came from northern latitudes.
• Hybrid: a cross of two or more of the above. Most retail flower is a hybrid.

These categories began as notes on plant form. Growers later applied them to effects. Genetic studies of hundreds of samples find that the labels do not track chemical output.

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Genotype and phenotype

Genotype is the DNA sequence in a plant. Phenotype is what the plant shows: height, leaf shape, bud density, resin, THC, CBD, terpenes. Environment acts on phenotype. Light cycle, temperature, nutrients, harvest day, and drying all change the final test numbers. Two clones from one mother share a genotype. Grow them in two rooms and the phenotype can differ.

Chemotypes

Researchers sort cannabis by chemical output, not by label. One common system uses four types.

1. Type I: THC dominant. CBD near zero.
2. Type II: mixed. THC and CBD both present.
3. Type III: CBD dominant. THC low.
4. Type IV: CBG dominant. THC and CBD low.

THC and CBD come from a shared pathway. CBGA is the starting compound. Enzymes convert CBGA into THCA or CBDA. A plant with active THCAS makes THCA. A plant with a nonworking THCAS gene makes CBDA instead.

How breeders move traits

1. Select parent plants with target traits.
2. Cross a female with a male, or cross two females with reversed pollen.
3. Grow the seed, test the offspring, and pick plants that carry the trait.
4. Backcross the pick to a parent to hold the trait and cut unwanted traits.
5. Self a plant to make S1 seeds, or cross two inbred lines to make an F1.
6. Repeat for several generations to reach a stable line.

A strain name is a label, not a guarantee. Two sellers can use one name for seed from different parents. A cut sold as clone only keeps one genotype. Seed from a cross varies.

What to check

Ask for a batch test with THC and CBD percentages, a terpene panel, and a harvest date. Terpenes such as myrcene, limonene, and caryophyllene appear on most panels. The ratio of THC to CBD and the terpene mix differ between batches of the same named strain.