Breeding Terms
Marijuana Horticulture
by Jorge Cervantes
In order to have a discussion on breeding, there are some terms we must learn in order to fully understand the concepts.
Genetic material is inherited as described above, in the seed making section, from both the pollen donor and mother plant. The genetic material, or deoxyribonucleic acid (DNA) is coiled into long, X-shaped strands called chromosomes and stored in the nucleus of every cell. In cannabis, each individual inherits 10 different chromosomes from the staminate pollen parent, and 10 different chromosomes from the seed mother or pistillate parent. The resulting individual has 10 chromosomes total, 2 copies of each of the 10 chromosomes, or 2 full genomes. This means there are two copies of every gene in the plant, one from the mother, and one from the father. Each and every cell in the plant has a copy of this unique DNA compliment. The genetic code is written along the length of the chromosome strands, and each gene has a specific location along its length.
Phenotype
We consider phenotype as the observable, qualifiable representation of a given trait. Anything you can measure, categorize, or otherwise observe in an individual can be considered a phenotype. Every plant has many different phenotypes. For example, plant height might be broken down into three categories or phenotypes: short, medium, and tall stature. There is a short phenotype, a medium phenotype, and a tall phenotype.
Cannabis flowers demonstrate different color phenotypes as well. Most often we see green calyxes, but there are also plants that have purple calyxes. Sometimes there are even green calyxes with purple markings. These are all different calyx-color phenotypes. There are also calyx-size and calyx-shape phenotypes, or leaf size and shape phenotypes. Every trait has different phenotypes that can be selected fr or against.
All phenotypes are the observable result of genes acting within the cells of the pant. Sometimes a singe gene controls one trait (monogenic traits), and sometimes sets of genes operate together and contribute to make what we see as a phenotype (polygenic traits).
Genotype
The genotype of a plant is a way of describing the actual genetic condition that results in the phenotype. As the genetic constitution or makeup of an individual, genotypes are not always expressed. Some are latent and only express themselves given the proper environmental stimulus. For example, some plants have green leaves, but the leaves will turn purple under cold conditions. Other green-leaved plants will not turn purple even under cold conditions.
This happens because these plants have a different version of the gene(s) that control whether purple pigments are to be produced in the leaves. These different gene versions are called alleles.
These plants initially both had the green-leaved phenotype, but one plant developed an altered phenotype (purple leaf) in response to an environmental condition. This is due t interaction of the genetics of the particular plant with respect to this trait (genotype) and the environment. A simplistic way to think of the concept is: Phenotype = Genotype + Environment.
Remember, this isn’t 100% true. More accurately, the phenotype(s) seen in a given individual are the result of an interaction of the plant’s genotype with the environment.
Let’s look at some possible corresponding genotypes in our short, medium, tall phenotype example. Remember, the genotype is our way of describing the genetic condition responsible for the phenotype, therefore we can assign it whatever values we want, it’s really just a symbol.
There are always 2 alleles, or versions of every gene, including the gene responsible for stature. When we have 2 “s” (lower case s) or “small stature” alleles, we see the short phenotype in the plant. Conversely, when the plant has 2 “S” (capital S) or tall alleles, the phenotypic outcome is a plant of tall stature. If the plant happens to inherit a copy of the tall and short allele, the resulting phenotype is a plant of medium stature.
Often, breeders base the symbol for the genotype on the first letter of the recessive expression of a trait. What this means will become clear over the next paragraph or so.
Homozygous / Heterozygous
These are terms used in describing the genotypic condition of a plant, with regard to the similarity of the alleles for a given trait. If a plant is homozygous for a given trait, it has two copies of the same allele (homo = same). If a plant is heterozygous, it has two different alleles for a given trait (hetero = different).