B-Locus: the spot in the genome that sets THC or CBD
In short
The B-Locus is the place in the cannabis genome where it is decided whether a plant makes mostly THC or mostly CBD. Two equally ranked variants sit at this spot. Which combination a plant inherits fixes its chemotype.
One enzyme gene, two variants
THC and CBD come from the same starting material, cannabigerolic acid (CBGA). Whether it becomes THC acid or CBD acid depends on which enzyme the plant makes: the THCA synthase or the CBDA synthase.
de Meijer and colleagues proposed a model in 2003 in which both enzymes are encoded by two variants (alleles) of a single locus, called B. The THC variant is BT, the CBD variant BD. The two are codominant, meaning neither suppresses the other. A plant with one of each makes both enzymes and therefore both cannabinoids.
Where the three chemotypes come from
From this model follows what growers see in practice. A pure THC plant carries BT/BT, a pure CBD plant BD/BD. Cross the two and the first generation is uniformly BT/BD, a mixed type. Mate those mixed types together and the second generation splits 1:2:1: a quarter pure THC type, half mixed, a quarter pure CBD type. de Meijer found exactly this ratio in every set of offspring examined and inferred single-locus inheritance from it.
Why it behaves like a single locus
Strictly speaking, THCA synthase and CBDA synthase are separate genes, as later genome assemblies showed. But they sit so close together and recombine so rarely that they are inherited as if they were one locus with two alleles. For breeding that is the key simplification: in practice you can treat the chemotype like a single codominant trait.
Limits of the model
The single-locus picture is robust but not the whole picture. Newer work found several copies and pseudogenes of the synthase genes in the genome and discusses whether more than one site is involved in cannabinoid formation. For the coarse THC-versus-CBD question the single-locus model holds; for finer differences in cannabinoid composition it falls short. So the B-Locus reliably explains whether THC or CBD dominates, not every nuance of the profile.