When you buy a particular strain of cannabis seeds, you expect to grow a plant with certain characteristics. For instance, you want your Sour Diesel to have a strong skunky taste while providing an energetic high, and you’d be disappointed if you got anything else. Commercial growers therefore have an obligation to ensure that their product is consistent and reliable, which means they need to breed plants with a high degree of genetic stability.
For smaller growers, this concept can be somewhat confusing and intimidating, but taking the time to get your head around the subject is certainly worthwhile if you want to raise your game.
What Is Genetic Stability?
In a nutshell, genetically stable cannabis strains are those that exhibit a high degree of uniformity and predictability, with minimal variation in characteristics between plants. In other words, genetic stability refers to the extent to which it is possible to predict the phenotypes that will be displayed in the plants that grow from your seeds.
As mentioned above, this is something of a gold standard for industrial breeders, as their customers want to be assured that the cannabis they are buying will always produce the same effects. It can take years to create genetically stable marijuana and the process is often very labour intensive, but it begins with a solid grasp of how genetics work.
Cannabis is a dioecious plant, which means that each individual is descended from separate male and female parents. As such, every cannabis plant has two copies of every gene, one of which it inherits from its mother while the other is passed down from the father. The two versions of a particular gene are known as alleles, and the way in which these alleles combine determines the physical characteristic, or phenotype, of a plant.
For example, let’s imagine there’s a gene that codes for flavour that can exist in two different forms. The allele for earthy flavour we’ll write as ‘E’, while the allele for diesel will be written as ‘D’. Remember that both parents have two alleles for this gene, and that each will pass one of these on to every offspring plant.
If both the mother and father plant have two copies of the E allele, then we know for certain that the next generation will receive two Es, and will therefore have an earthy flavour. However, if one or more parent possesses one or more D allele, then the offspring they produce can end up with a number of different combinations of alleles, which means their flavour might be earthy, diesel, or a mix of the two.
This can be expressed in the following way:
EE + EE = EE (earthy)
DD + DD = DD (diesel)
EE + DD = ED (mix)
ED + ED = EE or ED or DD (earth, mix or diesel)
And so on…
Yet things are often a fair bit more complex than this, as some alleles are dominant while others are recessive. In this instance, if we imagine that E is the dominant allele and D is the recessive allele, then E will always be expressed in the phenotype of a plant that has at least one copy of this allele. In other words, if a plant has an ED genotype, then it will have an earthy flavour with no traces of diesel. The only way a plant can have a diesel flavour is if it has a DD genotype.
Dominant alleles are typically denoted using capital letters while recessive alleles are usually represented by lower case letters. In this case, therefore, we would expect the following outcomes:
EE + EE = EE (earthy)
dd + dd = dd (diesel)
EE + dd = Ed (earthy)
Ed + Ed = EE or Ed or dd (earthy, earthy or diesel)
And so on…
Now, if you are trying to create a strain that will always have an earthy flavour, then you ideally only want to be crossing plants that have an EE genotype. As soon as you throw some d alleles into the mix you run the risk of ending up with a few diesel plants. Similarly, to create a diesel strain, you need to work with parents that have a dd genotype, as even a single E allele will result in earthy cannabis.
Plants that have two matching alleles for a given gene are called homozygotic, while those with two different alleles are called heterozygotic. Therefore, when breeders aim to improve the genetic stability of their cannabis, what they are really doing is trying to increase the proportion of homozygosis in its genotype.
In doing so, they raise the probability of a desired phenotype being expressed while reducing the chances of unwanted characteristics appearing in their weed.
Achieving Genetically Stable Cannabis
As previously stated, this isn’t an easy process and generally requires time, effort and a large growing space. In simplified terms, genetically stable cannabis can be achieved by inbreeding certain desired traits over numerous generations, although there are plenty of ins and outs that make the procedure a bit more complicated than that.
It all begins with a mother plant, which will be chosen because it possesses certain characteristics that a breeder wants to be predominant in the cultivar they are creating. It helps if this mother already has stable genetics, meaning it comes from a lineage that is known to possesses a high degree of homozygosis.
Cuttings will then be taken from this mother in order to produce numerous clones, all of which will be identical. These clones will then be bred with an unrelated yet genetically stable male plant that also possesses certain desired characteristics.
The resultant hybrid offspring are known as the filial-1, or F1, generation, and will display a fair amount of variation. This is because the alleles from the respective parent plants will have combined to produce a great deal of heterozygosis, resulting in a wide array of different genotypes and phenotypes.
Ideally, breeders want to propagate a huge number of F1 plants (hence the need for a large growing space) so that they can work with the full range of new genetics that they have just produced. Once these plants have reached maturity, those that display the most desirable characteristics will be crossed in order to give rise to the F2 generation.
However, because the F1 plants are already fairly heterozygotic, the number of new genetic combinations present in the F2 generation will be massive, which means these plants will be much less genetically stable than their parents and will therefore display a greater deal of variation. Selective breeding of large numbers of plants over many generations is therefore necessary in order stabilise a line. Often it takes at least five or six generations before results are seen, with up to 12 generations being required to create robust stability.
Fortunately, there are a few tricks that breeders can employ in order to speed up the process. One of the most common is back-crossing, which involves breeding a plant with a member of a previous generation rather than with one of its contemporaries. For example, by back-crossing an F1 plant with its mother, the resultant generation will be sure to contain more of the mother’s genetic make-up. This approach allows for the mother’s characteristics to become established in a new cannabis line fairly quickly.
Alternatively, some growers rely on selfing to reduce the amount of variation in their new strain. This is achieved by stressing the mother plant through the use of certain chemicals, causing her to grow stamen and become a hermaphrodite so that she can pollinate herself. As a consequence, the next generation will contain only the mother’s genes and will therefore be more likely to share her traits.
There’s Just One Problem…
You’re probably aware that inbreeding can be problematic in most species, so you won’t be surprised to hear that the same is true of cannabis. This is because repeated inbreeding narrows the gene pool of a population, which can lead to something called genetic depression.
When two genetically similar parents breed, their shared genotype becomes reinforced in the offspring. This means that any deleterious recessive alleles lurking in the family line will also be bred into this new generation. If two members of this generation are then crossed, the chances of them both passing on this undesired recessive allele are increased, which means that the phenotype associated with it will sooner or later become expressed in the lineage.
When this occurs, it means it’s time to start thinking about outbreeding. Introducing an unrelated cannabis plant into the mix will result in greater genetic diversity within the line, and will usually resolve the problem of genetic depression within a few generations.
Given the numerous complexities associated with stabilising cannabis genetics, it’s generally not the sort of thing that small-scale home growers ever attempt to achieve. Fortunately, however, many excellent breeders have been on the case for several decades, which is why there are so many reliable cultivars on the market today.