An allele is one version of a gene. Cannabis is diploid, so every plant carries two alleles at each locus, one inherited through the pollen and one through the ovule. If the two are the same the plant is homozygous at that locus; if they differ it is heterozygous.
In practice
The best-known cannabis example is the cannabinoid synthase locus. One allele (often written B_T) makes a functional THCA synthase; another (B_D) makes CBDA synthase. A plant carrying B_T/B_T is THC-dominant, B_D/B_D is CBD-dominant, and B_T/B_D produces both in a roughly balanced ratio (de Meijer et al. 2003). Breeders who want a stable 1:1 THC:CBD seed line learn quickly why it cannot breed true: the balanced plants are heterozygous and their offspring segregate.
Alleles are written with letters (A and a) for teaching. Real cannabis loci can have more than two alleles in the population, and some regions contain duplicated, non-functional copies of synthase genes (Laverty et al. 2019).
Don't confuse with
- Gene: the position and function; the allele is the variant at that position.
- Mutation: the event that creates a new allele. Most alleles in a population are old, not new.
- Phenotype: what you see. Two different allele pairs can produce the same visible result when one is dominant.
A gene is a stretch of DNA that carries the instructions for one functional product, usually a protein such as an enzyme, and is the basic unit of inheritance.A locus is the fixed position on a chromosome where a particular gene or DNA marker sits; the plural is loci.A plant is heterozygous at a locus when it carries two different alleles there (Aa), so its offspring can inherit either one and segregate.THCA synthase is the trichome enzyme that converts CBGA into THCA; which alleles of its gene a plant carries largely decides whether it is THC-dominant.CBDA synthase is the trichome enzyme that converts CBGA into CBDA; it competes with THCA synthase, and its alleles largely decide whether a plant is CBD-dominant.THC (Δ9-tetrahydrocannabinol) is the neutral, decarboxylated cannabinoid mainly responsible for cannabis's reported intoxicating effects; the living plant makes it almost entirely as THCA.CBD (cannabidiol) is a non-intoxicating neutral cannabinoid formed when CBDA decarboxylates; hemp-type and CBD-dominant cannabis make it mainly as CBDA in their trichomes.A dominant allele is one whose effect shows in the phenotype even when only one copy is present, masking the other (recessive) allele at that locus.A plant is homozygous at a locus when it carries two identical alleles there (AA or aa), so it passes the same allele to every offspring.A recessive allele only shows its effect when a plant carries two copies of it; in a single copy it is masked by a dominant allele and carried silently.
