Epistasis is when the effect of one gene depends on which alleles are present at another gene. One gene can mask another, enhance it, or only matter in combination with it. Dominance is an interaction between alleles at one locus; epistasis is between loci.
In practice
Pathway genes give the cleanest examples. A cannabinoid synthase allele cannot do anything if the plant lacks enough of the precursor CBGA upstream; that is part of how chemotype IV (CBG-dominant) and chemotype V (cannabinoid-free) plants arise (de Meijer et al. 2009). The synthase genotype only shows when the upstream genes work.
For breeders, epistasis explains why a great combination sometimes vanishes in the next generation: the good result depended on alleles at two or more genes being together, and segregation split them. It is also one reason an elite clone can be easier to keep than to recreate from seed.
Classic modified F2 ratios signal epistasis: 9:3:4 (recessive epistasis), 12:3:1 (dominant epistasis), 9:7 (complementary genes), instead of the 9:3:3:1 of two independent genes.
Don't confuse with
- Dominance: within one gene.
- Pleiotropy: one gene affecting several traits.
- Linkage: two genes inherited together because of their position, not because they interact.
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.An allele is one version of a gene; a diploid cannabis plant carries two alleles at each locus, one from each parent, which may be the same or different.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 cannabinoid is any compound that binds the body's cannabinoid receptors or shares the typical cannabis terpenophenolic structure; the plant-made ones are phytocannabinoids such as THCA and CBDA.A chemotype is a plant's chemical phenotype, in cannabis usually its dominant cannabinoid class: THC-dominant (I), mixed (II), CBD-dominant (III), CBG-dominant (IV) or cannabinoid-free (V).Segregation is the separation of a plant's two alleles at each locus into different gametes, so offspring of a heterozygous parent vary in which allele they get.An elite cut is a famous, proven clone-only plant, often held by a few growers and traded rather than sold openly.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.A quantitative trait varies continuously (yield, height, flowering time, cannabinoid content) because it is controlled by many genes of small effect plus the environment.Heterosis (hybrid vigour) is the tendency of offspring from genetically different parents to outgrow or outperform both parents in traits such as vigour and yield.Pleiotropy is when a single gene affects two or more apparently unrelated traits, so selecting on one trait changes the others too.
