What's a Genotype? What's a Phenotype?

By Mitchell Colbert

This is a post for the serious cannabis breeder who isn't afraid of a little science-talk. By the end of this post you'll have a solid working knowledge of what makes your plants do what they do, grow like they do, and might have some new ideas about how to grow them better than you are already. What's even better is that everything you are about to read is applicable not just to cannabis but to growing all other plants, dog-breeding, and all kinds of other things. We're talking genetics, genotypes and phenotypes!


Let's begin with the basics, what makes some plants tall and others short. In the cannabis world, there are three main sub-species of cannabis; indica, sativa, and ruderalis. While recent research shows these might be inaccurate, let's stick to them for now to keep things simple. Plants of the cannabis sativa variety usually are taller and skinnier, with wider spaces between the leaves than their shorter and stockier indica cousins. When processed, indica buds tend to be much denser than sativa buds. Ruderalis is a so-called 'wild' species of cannabis, that unlike the other two sub-species, does not depend on light cycles to flower and instead automatically goes into a flowering cycle after a given number of days. This is a genotypical view of cannabis, where one only considers the genetic ancestry of the plant. Plant genetics are a topic of endless debate and current research has just made the debate even more heated. For this post, let's consider indica, sativa, and ruderalis as phenotypical descriptions that tell us how to expect a plant to look and behave.


But before we get into the meat of this post, what is a genotype and what is a phenotype?


Genotype: An organism's genotype is its heritable genetic identity. This is what it gets from its parents and what would be shown by viewing its genome sequence.

Phenotype: Something's phenotype is its outward physical manifestation, like size, shape, and color. The phenotype includes all physical parts, and the sum of all atoms, molecules, cells, observable structures, tissues, behaviors, functions, and chemical properties.


For people, the genotype is what comes from your parents and you have no control over, the phenotype is how you choose to express yourself, based off the context of your environment and upbringing. Maybe you are born the child of rich yuppies, born to be a lawyer and also be a rich yuppie, but perhaps you get to college and discover punk music, shave your head into a mohawk, and rebel. Phenotypical changes can do a complete one-eighty flip on the genetic make up of a person or a plant. For example, you have a plant that was meant to be a tall sativa, something like Durban Poison, but perhaps you accidentally do not give it enough phosphorous. A phosphorous deficiency can stunt the growth of a plant, making them shorter and stockier, more like an indica and less like a sativa.


Genotypes and phenotypes express themselves according to the Law of Dominance, which was discovered by a Augustinian friar by the name of Gregor Mendel. Mendel spent a lot of time observing pea plants in his garden and as a result taught modern botanists the core of everything we still know and use today. The Law of Dominance is Mendel's first law of inheritability, and it holdss that recesive traits will be masked by dominant traits. Mendel's second law is the Law of Segregation which describes what is depicted by a Punnett Square. Mendel's third and final law, the Law of Independent Assortment, says that all genotypical and phenotypical expressions are totally independent from one another. Basically, the genes or phenotype that controls what color a plant's leaves are will be different than those that control how much THC or CBD it produces. See the example below of the Punnett Square which shows what THC/cbd dominance should theoretically look like for the majority of plants.


THC-CBD Dominant vs Recessive Chart

This chart shows that 25% of your plants should theoretically have a THC-rich profile, expressing a heavily THC dominant profile. Another 50% should have a more balanced ratio, and the remaining 25% will be showing a double recessive trait for CBD-dominance rather than THC. Unlike the color of the flowers for Mendel's peas, THC dominance has been seen as a desired trait since at least 1964 when Raphael Mechoulam first discovered it, if not even further back in cannabis' history. Because of that breeders have selectively bred for more THC over time, at the expense of other cannabinoids like CBD. This has created a skewed Punnett Square where closer to 75% of offspring are THC-rich, and only 25% are CBD-rich. This can be selectively breed back up to be more similar to what is expected by the Punnett Square, with 25% THC-rich dominants, 25% CBD-rich recessives, and 50% being hybrid offspring with both a dominant and recessive trait yielding about a 2:1 CBD-THC ratio.


That 25% rate of CBD-dominance is not strain specific and holds true of most CBD-rich strains, but some exceptions do exist. This means that anyone trying to pop CBD-rich strains from seeds needs to be aware that many of those seedlings may be THC-rich rather than CBD-rich, the only way to know is to get your seedlings tested. I have seen samples of Shark Shock and Harlequin that have more CBD than THC, others with the reverse; similarly I have heard horror stories from doctors whose patients wanted CBD-rich ACDC oil and ended up with something THC-rich. The fact that the majority of offspring of CBD-rich strains will actually be THC-rich is further evidence that what people name strains means a lot less than what the resulting bud lab tests at.