Cannabis Science

7 May 2021

How do Cannabis Genetics Work?

Growers just starting their cannabis cultivation journey are likely to come across some technical language that is difficult to understand and interpret. Terms such as sativa, indica, landrace, ruderalis, “genotype”, and “phenotype” are barriers to entry to the hobby grower who is simply trying to find seeds they can put in pots under grow lights. The field of cannabis genetics has something for every level of grower. 

Read on to find out more about cannabis genetics terminology and its application in practice.

Growth habits, not effects or species

Definitions and models are constantly being updated in science, as anyone keeping track of the development of perceptions of the medicinal, societal and recreational value of cannabis will be able to say. One fallacy in need of updating that has existed in the cannabis community for some time is the association of the terms sativa and indica with the psychoactive effects of those particular plants. Growers seed shopping may hear that sativa strains provide active, elevating, and euphoric effects ideal for socialising and high energy activities while indica strains sedate and relax.

These two terms were established in the 18th century to denote what was thought to be two different species of the cannabis plant, based on their recognisable physical traits and growth habits. Cannabis sativa is characterised by bright, vibrant green foliage, tall growth habit, narrow leaves, and a flowering time of up to three months. Cannabis indica features dark-green foliage, a shorter, bushy growth habit, wide-fingered leaves, and a two-month flowering period, with denser flowers more laden with resin.

Over the decades, these terms became associated with the psychoactive effects produced by inhaling or ingesting cannabis. Molecular diagnostic testing has since shown that there is no difference between sativa and indica on a genetic level. The immense variability of traits (scent, flavour, medicinal and psychoactive effects) expressed by the cannabis flower is determined mainly by the growing environment and the plant’s genetic base. The therapeutic and psychoactive effects are derived from the plant’s naturally occurring compounds, which are produced in response to environmental stimuli.

These compounds, also known as cannabinoids, are found in glands on the epidermis or the plant’s outermost layer of cells. The glands, or trichomes, also contain terpenes which are non-psychoactive essential oils that help determine the smell and flavour of each strain. Cannabis plants develop these compounds as defence mechanisms to deter anything with an urge to chew on flowers or leaves.

It is more helpful to think of sativa and indica as words to describe the growth habit of a cannabis plant rather than its flowers’ psychoactive effect or its species. They are horticultural terms that identify, for example, whether a plant is going to grow short and bushy, with tight bud structure on its flowers that finish earlier (also known as indica-dominant), or tall and leggy with a later finishing larger and airy flower (sativa-dominant).

As such, while the terms may have been proved inaccurate as descriptors of species, they still have value for informing the cultivator of what kind of physical characteristics to expect from their plants. Decades of selective breeding by cultivators attempting to stabilise plant genetics or discover new, exciting combinations have led to plants containing both sativa and indica genetics. These are known as hybrids, which is a term the grower shopping for seeds will come across.

In the 20th century, a third cannabis species, ruderalis, was observed. Characterised by a smaller, thinner growth habit and a life cycle that isn’t determined by light schedule, ruderalis begins flowering once they are between five and seven weeks old. Ruderalis genetics have been crossbred with sativa and indica to produce “auto-flowering” strains, combining the more concentrated cannabinoid and terpene profiles of sativa and indica cultivars with the maturity-based life cycle of ruderalis.

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Genotypes, phenotypes, chemotypes

The grower diving deeper into cannabis genetics will come across the terms “genotype”, “phenotype”, and even “chemotype”. These are derived from genetics terminology and are different ways for cannabis breeders to describe the traits carried by and expressed by cannabis plants.

As mentioned above, the two largest determinants of how cannabis plants grow are the growing environment and the plant’s genetics. The plant’s genetic composition, or genotype, allows it to express a range of growth possibilities, but the growing environment is what brings out particular dominant characteristics.

The phenotype is the physical expression of a genotype or the traits that the environment induces the plant’s genetics to develop. The environment determines various plant properties, including shape, colour, smell, and resin production. As such, if two growers were given identical clones, each final result will be different as the growers will have had their own grow environments even if the identical clones possessed the same genetic instructions.

The modern cannabis industry has even classified cultivars by chemical composition or chemotype. This is a marked development along the timeline of cannabis breeding and identification, a step of progress beyond classification solely by sativa and indica characteristics and terpene profiles. Chemotypes of cannabis cultivars can be classified into three categories of cannabinoid concentration: THC-predominant (Type I), balanced THC/CBD concentration (Type II), or CBD-predominant (Type III).

Classification by chemotype is one way to better define the traits and characteristics of a plant that can have hundreds of different expressions of its genetics that have been bred for centuries. As public perception of cannabis matures to recognise its value, cultivar identification by chemotype and concentration of medicinally valuable compounds will become even more important.

Hand holding cannabis plant while examining cannabis genetics and what they mean

Genetics in practice

The tried-and-tested method for preserving the desirable genetics of a cannabis plant is to clone it by the propagation of cuttings or tissue culture micropropagation. This ensures genetic fidelity – identical clones of the mother plant are regenerated from plant tissue. This is how famous cannabis strains such as Super Lemon Haze or Gorilla Glue #4 came to be cultivated by growers worldwide.

Unless growing using clones, the grower hunting for desirable phenotypes using seeds should expect a wide range of phenotypes to be expressed by their plants. Expressions of genotypic traits such as vigour, height, scent, taste, pest resistance, and bud structure will exhibit on different levels on different plants.

In a pack of ten seeds, there may be as many as four different phenotypic expressions of a cannabis strain. A grower crosses cannabis plants by taking pollen from a male plant of one strain and placing it in contact with the female flower of a different strain, pollinating the flower and producing seeds containing genetics from both plants. The resulting seeds will grow into plants that, generally, contain the same genetic information with a few crucial differences that may or may not express strongly in the growing environment.

Outlier plants, such as particular plants that exhibit the most vigorous growth, exemplary bud formation, pest resistance, strongest concentration of cannabinoids (including THC levels), loudest or most unique terpenes, become “champion” plants that the grower should consider and continue producing, or perhaps breed with other outstanding plants.

When two different genotypes of plants are crossed, the resulting genetics are known as the F1 or first-filial generation, which is the most genetically stable and highest-yielding generation. F2 generations are produced when two F1s are crossed with each other. F3s are produced when two F2s are crossed, and so on for any subsequent generations.

Inbred lines or IBLs describe any plants from F5 onwards which result from selective inbreeding. Generally speaking, the traits found in the F5 generation or after are so dominant that they express almost as an entirely different set of traits than those of the F1s the IBLs came from. Growers should consider that the higher the F-number, the likelihood of variation in the resulting generations increases, and the likelihood of parents passing on their desired traits are reduced.

Backcrossing cannabis strains is when the grower crosses an F1 hybrid strain and breeds it with an original parent plant. The resulting seeds are known as BX1. Backcrossing cannabis plants is how growers traditionally stabilise or preserve particular desired traits for future production.

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Post author
Martin
Martin is a production horticulturist with experience in commercial cannabis cultivation and sustainable farming from his time with Emerald Cup Award-winning farmers Esensia Gardens in northern California's Emerald Triangle.
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