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Why Aren’t All Sibling Cannabis Plants Identical?

Whether you’re a breeder trying to create a killer new hybrid or a home grower planting your favourite cultivar, chances are you’ve noticed that your cannabis plants don’t all come out identical. The reasons for this will depend primarily on how stabilised your genetics are, although environmental conditions also have a massive role to play. At the end of the day, a complex tapestry of factors will determine what sort of weed you get, so while your seeds might share the same name, the reality is that sibling cannabis plants aren’t always identical.

Cannabis Plants Don’t Always Turn Out Identical

Before we get into the nitty-gritty of breeding and genetics, it’s worth clearing a few things up about how cannabis plants can be affected by the environment, even if they’re from a highly stabilised cultivar.

Seeds from such lines tend to be highly homozygous, meaning they all have very similar genetics, with little variation. Yet a plant’s phenotype – or the actual characteristics it possesses – isn’t determined solely by its genes.

For example, if you were to plant two seeds from a cultivar that is known for its purple colouration – like Purple Kush, for instance – then you’ll generally find that cooler temperatures during flowering bring out this hue much more vividly than warmer environments do. This is because cannabis plants tend to devote less energy to chlorophyll production when the temperature drops, thereby allowing other colours to shine through.

Thus, while all plants from a particular line may carry the genetic potential for purple bud, the degree to which these genes are reflected in plant’s phenotype often depends on external factors. In other words, cannabis plants from the same cultivar aren’t guaranteed to be identical.

Let’s Get Stuck Into The Genetics

Variation between individuals occurs because every gene exists in an array of different guises. These distinct versions of a gene are called alleles, and every plant has its own unique combination of alleles. For example, if we imagine that a single gene controls THC production, then a plant could carry one allele for high THC production and one for low THC production. Bear in mind that each plant carries two alleles for every gene, one of which is inherited from each parent.

Some alleles are dominant, which means they are always expressed, while others are recessive, and will only be expressed in the absence of a dominant allele. In the above example, the actual amount of THC will depend on the interaction between the two alleles, along with certain environmental factors. In reality there’s much more to it than this, but the basic point here is that just because a plant carries an allele for a particular phenotype, doesn’t mean that characteristic will always be expressed.

How This Affects Breeding

A deeper understanding of what causes cannabis plants to be identical – or not, as the case may be – is required once you start thinking about breeding. Stabilising a genetic line is a long and difficult process, which we’ve covered in previous blogs. In short, you start off by crossing a male and a female from different strains in order to produce a hybrid, which must then be repeatedly inbred in order to strengthen the desired genetic traits and create a line of cannabis plants that are as close as possible to identical.

The initial cross gives rise to what is known as the F1 generation, with the next round of cross-pollination creating the F2 generation, and so on. What we’re interested in talking about here, though, is how come variation increases in the first few generations of offspring, before decreasing once several rounds of inbreeding have been completed. In other words, why do the F1 and F2 generations contain so few cannabis plants that are identical?

The answer is down to a process called meiosis, which results in the formation of sex cells – or pollen and ovule, in the case of cannabis. During meiosis, a diploid cell – which contains two copies of every chromosome – divides twice to form four haploid cells, each of which contains only one copy of each chromosome. These haploids are the sex cells, and eventually combine with one another when pollen meets an ovule during sexual reproduction, giving rise to a seed that contains two sets of chromosomes.

Before a diploid cell splits in two, its homologous chromosomes line up with one another and exchange some of their alleles, in a process known as crossing over. By the time they separate, each chromosome will have developed a new arrangement of alleles, making it genetically unique. This rearrangement of chromosomes is referred to as recombination, and is the main reason why sibling cannabis plants aren’t identical.

A more detailed explanation of the meiosis process. Image: Natalieconstancehall/Wikimedia Commons

When two plants are crossed, large numbers of genetically unique haploids pair up, giving rise to a highly heterozygous F1 generation that contains a huge variety of different combinations of alleles. These plants are then crossed again in order to produce even more novel configurations of alleles in the F2 generation.

Over many generations, breeders will carefully select and cross the plants that most strongly express their desired phenotypes, reducing the amount of variation and ultimately creating a stable line of more or less genetically identical seeds.

Until that point is reached, however, breeders will repeatedly notice that their sibling cannabis plants are anything but identical.

Cultivation information, and media is given for those of our clients who live in countries where cannabis cultivation is decriminalised or legal, or to those that operate within a licensed model. We encourage all readers to be aware of their local laws and to ensure they do not break them.

This post is also available in: French

Ben Taub