Breeding

The process of crossbreeding plants to produce desirable traits. As this has become the backbone of the cannabis industry, the selection process and the terminology surrounding it deserve closer examination to inform consumers.

Learn more about selecting cannabis strains

To understand selection, you need to understand the mechanisms of plant reproduction. Cannabis has a diploid genome, which means it has two copies of each chromosome, where the genes are located. During reproduction, a male plant and a female plant are crossed with one another, allowing each to contribute genetic information to their offspring.

A copy of each parent plant’s chromosome is provided by that plant’s sex cell—pollen in male cannabis plants and the ovule in female cannabis plants. These cells combine during reproduction, mixing genes from one parent’s chromosome with regions of the other parent’s chromosome in a process called recombination.

The recombination of genetic material from each parent plant during reproduction is very beneficial to the health of future generations. It allows genetic material to cluster in a specific region of the chromosome, which facilitates the selection or elimination of these traits in future offspring.

This potential for increasing beneficial traits that promote the organism’s survival makes selection advantageous for the longevity and health of a lineage, which can result in a better-adapted cultivar or variety.

Why is selection useful?

This complex process of selecting desirable traits requires a great deal of time, effort, and resources on the part of the breeder. Cannabis breeding as practiced today began in the 1970s. From the outset, one of the main goals of breeding was to increase the potency of cannabis, which initially contained only 3 to 5 % of THC by weight. The potency of cannabis has steadily increased to 17–25% in modern strains, but other traits have been more difficult to achieve. Modern breeders seek a unique aroma to set their product apart from the competition. Varieties that promote resistance to pests are sought after, as molds and insects adapt to feed on cannabis.

The iterative selection process takes place over several generations. A plant with a shorter flowering time but that is susceptible to pests could be crossed with a plant known to be resistant to a variety of pests. The first generation of offspring resulting from an initial cross between two varieties is called an F1 hybrid; some may exhibit greater resistance to pests than others. The plants can then be crossed again to further enhance these traits, producing a second generation, or F2 hybrids.

This process can be repeated, using the best plants from each generation to selectively fix these traits. A breeder can conclude that the traits are fixed in their hybrid line after several cycles of crossing the best male and female hybrids from each crop when subsequent generations do not differ in the expression of the targeted traits observed in previous generations. At this stage, the genetics of the hybrid line are considered stable and should produce plants identical to the stabilized hybrid parents.

Once a variety’s genetics have been stabilized, its seeds can be sold to individuals who wish to grow a similar plant. However, these individuals often live in very different climates, which can affect the outcome of the crop. Initially, the seeds are simply planted in the new climate. Among these newly grown plants, those that have adapted best to their environmental conditions will be crossed with one another to produce seeds that are best suited to their new environment.

Types of Selection

To align with a producer’s goals for the final product, two types of selection are used: inbred lines and hybrid lines.

Inbred lines

So-called inbred lines result from crossing a large population of plants of the same variety. This form of reproduction homogenizes desirable traits but reduces genetic diversity, since all generations will be related to one another. Self-pollination, which is the most extreme form of inbreeding, occurs when a plant pollinates itself, producing offspring with identical genetic makeup.

These inbreeding methods result in little or no recombination, causing the lines to become less vigorous over time—a phenomenon colloquially known as «genetic stagnation.» Outbreeding and backcrossing, as explained below, are other types of breeding methods that can restore diversity and vigor to these lines.

Durban Poison, a popular strain available at many dispensaries, is an excellent example of an inbred strain with consistent characteristics. The sweet aroma characteristic of this strain is clearly evident when comparing two bags or extracts of it.

Hybrid lines

In the case of outbreeding between distinct lines, true inbred lines that exhibit favorable traits but significantly lower vigor can be crossed with one another to produce a hybrid line. Hybrid lines allow for the recombination of very different genetic information, which increases the genetic diversity and health of the line—a trait often referred to as hybrid vigor. Unfortunately, F1 hybrids may exhibit some of the parents’ traits, but not to the same degree, or they may exhibit new and undesirable traits. Backcrossing—which involves crossing the F1 generation with the parental plants—can be used to reinforce parental traits in more diverse hybrids.

Tangie is another popular and widely available strain that is an example of a hybrid line. Developed by Crockett Family Farms through a cross between Cali-O and Skunk, this strain is widely sold by the seed company DNA Genetics. Growers who purchase these seeds often notice a wide variety of phenotypic traits—such as different plant heights and leaf shapes—resulting from a bag of seeds that all share the same genotype Tangie. Despite their differences, all these plants share the characteristic mandarin aroma.

Breeding, phenotype selection, and elite clones

Breeding and the phenotype hunting are inseparable. A breeder who creates a new F1 hybrid always germinates a large number of seeds to identify the best phenotypes from among the natural variability of that generation. It is through this process that the elite clones — these exceptional phenotypes, preserved indefinitely through cloning, which become genetic benchmarks within the community.

The landrace varieties — these non-hybridized original strains, such as Hindu Kush, Lamb’s Bread, and Durban Poison — form the historical foundation of modern breeding. They are the «P» (parent strains) from which all contemporary hybridization began in the 1970s and 1980s, particularly in the Dutch and Californian scenes.