The Genetics Of Marijuana

genetics of marijuanaThe mix of cannabinoids in a particular strain of marijuana is largely a product of heredity. By contrast, the total level of cannabinoids is dictated both by genetics – some strains can achieve higher levels than others – and by how well the marijuana has been grown.

It appears that a single gene dictates whether a plant produces THC or CBD. A plant inherits a copy of this gene from both of its parents. If both genes are of the THC type, the plant produces almost entirely THC; if CBD type, almost all CBD, and if the plant inherits one of each, it will produce a mix of THC and CBD. There is significant variation in the “strength” of these genes, so the ratio of THC to CBD production varies from plant to plant depending on the exact copy of the genes the plant has inherited. Each clone of the plant will, however, produce essentially the same ratio of THC to CBD as its “parent”.

In hemp strains, the CBD form tends to predominate, so the primary cannabinoid in hemp is CBD. By contrast, drug strains of marijuana have been selected over centuries of breeding for the THC gene.

Indica and Sativa

There is an enormous amount of contradictory and confusing information about what “indica” and “sativa” types are and what medical properties correspond to these categories. The confusion is not just among the marijuana community; the scientific community can’t agree, either. Various researchers say cannabis is one, two, or three different species; some divide cannabis into two species, indica and sativa, but then put all drug types – including those we think of as ‘indica’ and ‘sativa’ – into the indica category.

What little agreement there is:  some marijuana strains are tall and skinny, with narrow, light green leaves. These are generally assigned by the medical marijuana community to the “sativa” category, and appear to have been bred from the hemp brought to Europe centuries ago from Africa. Other marijuana strains are bushier, with wider, dark green leaves. These are assigned to the “indica” category and seem to have originated in India and the middle east. Indicas generally are grown indoors, have higher yields than sativas and are usually easier to grow.

Sativa drug strains appear to be a very highly bred subgroup of cannabis plants; they have been selected for generations for only the strongest versions of the THC gene, and the CBD gene has been virtually eliminated. They are the purebreds of the marijuana world; the strains have identifiable characteristics, and if you cross two plants of the same strain, the seeds will run relatively true to the parent.

Indica drug strains, by contrast, seem to have been less highly inbred; there is more genetic variation within and between strains, and the CBD gene is still present in many strains. Seeds are less likely to run true to the parents, and the results of crossbreeding are less certain (and more potentially interesting) than crosses between sativa strains. High levels of THCV are also more common among indica strains.

Finally, there are some strains reported in the literature with very high levels of THCV, CBDV or CBG, with correspondingly lower levels of THC and CBD; we have yet to encounter any of these varieties in our testing.

For further reading:
de Meijer et al., 2003, The Inheritance of Chemical Phenotype in Cannabis sativa L., Genetics 163:335-346.

Hillig and Mahlberg, 2004, A Chemotaxanomic Analysis of Cannabinoid Variation in Cannabis (Cannabaceae), American Journal of Botany 91(6):966-975