Two alleles are codominant when a heterozygous plant expresses both of them rather than one masking the other. Incomplete dominance is a close relative: the heterozygote is intermediate between the two homozygotes. Growers usually lump the two together.
In practice
The cannabinoid synthase locus is the standard cannabis case. In de Meijer's model, B_T codes for THCA synthase and B_D for CBDA synthase. B_T/B_T plants are THC-dominant (chemotype I), B_D/B_D plants CBD-dominant (chemotype III), and B_T/B_D plants make both, typically in a ratio near 1:1 (chemotype II) (de Meijer et al. 2003).
This has a direct breeding consequence. Cross two balanced 1:1 plants and the offspring segregate about 1 THC-dominant : 2 balanced : 1 CBD-dominant. A "1:1 seed line" cannot be fixed by selection alone, because the balanced type is the heterozygote. The stable route is to cross a THC-dominant and a CBD-dominant parent, which gives uniform balanced F1s.
Expected offspring of B_T/B_D × B_T/B_D: 25% B_T/B_T, 50% B_T/B_D, 25% B_D/B_D. Real ratios vary because the region also contains duplicated and non-functional synthase copies (Laverty et al. 2019).
Don't confuse with
- Dominant: one allele masks the other; in codominance neither is hidden.
- Ratio inheritance: the exact THC:CBD ratio within balanced plants still varies with other genes and environment.
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 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.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.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).F1 is the first generation of offspring from crossing two parents; if the parents are inbred and different, F1 plants are uniform and often show hybrid vigour.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.The chemotype ratio is the balance of a plant's main cannabinoids, usually THC to CBD, classing plants as type I (THC-dominant), II (mixed), III (CBD-dominant), IV (CBG) or V (near-zero).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 plant is heterozygous at a locus when it carries two different alleles there (Aa), so its offspring can inherit either one and segregate.
