For simplicity sake, let’s define genetics as the study of genes and say that genes code for proteins. These of course are not absolutes, as many examples exist which defy the above statements. Nevertheless, a vast majority of genetics involves genes that code for some type of protein, the genotype, and those proteins function to provide an observable outcome within the organism, the phenotype.
Genes exist as a ‘code’ within deoxyribonucleic acids (DNA). Four nucleotides make up the building blocks of DNA, Adenine (A), Cytosine (C), Guanine (G) and Thymine (T) and a codon of three deoxyribonucleic acids codes for individual amino acids. For example, the codon ATG codes for the amino acid methionine.
In order for a gene to be turned ‘on’ or expressed, it must do so through a two-step process. The first step, known as transcription, involves copying a DNA sequence into an intermediary message of ribonucleic acids (RNA). That ‘messenger RNA’ is then translated into a protein and that protein subsequently goes out and performs its function.
An Augustinian priest by the name of Gregor Mendel (1822-1884) first described the observable effects of genes in pea plants. Through incredibly well controlled breeding experiments, he isolated what he defined as ‘factors’. Those factors, or genes as we now know them, were responsible for the phenotypes he bred; tall plants vs. short plants, green peas vs. yellow peas, wrinkled peas vs. smooth peas…
Through his careful selection of plant phenotypes and his extreme attention to detail, we now attribute the father of genetics title to Gregor Mendel.
By breeding and observing the inheritance pattern of specific plants, Mendel discovered dominant and recessive genes. When he crossed a pea plant with yellow peas (dominant) to a pea plant with green peas (recessive), all the offspring produced only yellow peas. However, when this ‘yellow’ pea plant generation was bred upon itself (F1 intercross), the resulting offspring possessed yellow and green peas in a ratio of 3:1, respectively.
This simple breeding experiment demonstrated pea color was controlled by one gene even though variation exists (green vs. yellow peas). When different variants of a gene exist, we define those variants as alleles.
Blood type in humans uses one gene with three different alleles allowing for the possibility of four different blood types; A, B, AB and O. When you add in the Rh factor (O+ vs. O-) the combination of two genes with multiple alleles allows for even more variation.
Take Labrador retrievers as another example, with the phenotypic coat colors of black, yellow and chocolate. The three different coat colors occur through the interactions of two genes. The first gene known as the ‘extension trait’ determines whether or not the dog’s coat will possess color. When the dominant allele is present in either the homozygous (EE) or heterozygous state (Ee), the fur will have color and the dog will be black or chocolate. However, when the recessive allele is homozygous (ee), the fur will lack color and you will see a yellow dog. Therefore, yellow Labrador retrievers are a product of the lack of coat coloring.
When present, the dominant E allele will allow for coat coloring to occur, but the presence of the black or chocolate coloring is determined by the ‘brown trait’.
This gene affects color pigmentation and when the dominant allele is present (BB or Bb), the dog will be black, but when the recessive allele is present (bb), the dog will be chocolate. Therefore, chocolate labs are a product of coat coloring without the ability to make black (EEbb or Eebb).
Things become even more complicated as more genes get involved. In the mouse, coat coloring is determined by at least five genes (A, B, C, D & P) and the combinations of different alleles can lead to coat colorings such as white, black, albino, silver, fawn, yellow, champagne, cream and many more…
Gregor Mendel performed his studies on genes that possessed dominant and recessive alleles, but as it were, not all genes utilize dominant and recessive alleles. Back to blood type, the A and B alleles are dominant to the O allele, but the A allele and B allele are ‘codominant’ to one another in that they both produce a product, which is how someone can possess an AB blood type.
So how does all this relate to Cannabis?
To put it simply, genetics control the cannabinoids. The gene that makes THC is called THC Synthase; CBD is made by CBD Synthase, CBG by CBG Synthase and so forth for most if not all of the various cannabinoids made by the plant.
Exactly how these genes can result in plants producing under 0.3% THC (hemp) while others produce as much as 30% THC (marijuana) will be the topic of discussion in next week’s post.