Short answer
THC and CBD plants are the same species with a different enzyme at one fork in the pathway. A THC-dominant plant turns the precursor CBGA into THCA; a CBD-dominant plant turns it into CBDA; a mixed plant does both. The trait inherits as one near-codominant block. Established genetics.
Why
One precursor, competing enzymes. CBGA sits at a fork, and THCA synthase and CBDA synthase compete for it. Crossing pure THC and pure CBD plants gave uniformly mixed F1s and a 1:2:1 split in the F2, explained by one locus with codominant alleles [1].
The "locus" is really a gene block. The two synthase genes sit in large, repeat-rich, low-recombination regions, so they pass on together [2]. CBD in drug-type lines often traces to hemp bred into marijuana backgrounds [3].
Seedlings can be screened. Validated DNA markers identify chemotype from young plants [4], weeks before flower.
How much cannabinoid a plant makes is a separate, many-gene trait. Chemotype sets the ratio, not the potency.
The numbers
| Chemotype |
Ratio |
Synthase alleles |
Breeds true? |
| I, THC-dominant |
THC:CBD > ~10:1 |
THC / THC |
Yes |
| II, mixed |
~0.5:1 to 3:1 |
THC / CBD |
No: 1:2:1 |
| III, CBD-dominant |
CBD:THC > ~10:1 (hemp often 20–30:1) |
CBD / CBD |
Yes |
| IV, CBG-dominant |
High CBG |
Non-functional, both copies |
Yes, once homozygous |
| Cross |
Offspring |
| I x III |
100% II |
| II x II |
25% I : 50% II : 25% III |
| II x III |
50% II : 50% III |
What to do
- Want a 1:1 plant? Keep it as a clone; its seed won't hold the ratio.
- Breeding CBD into a THC line? Cross, then select type III in the F2 or backcross.
- Screen seedlings with a chemotype test, then confirm with flower tests.
- Growing CBD as hemp? Test before harvest; rules vary, so check local limits.
Common mistakes
- Expecting a 1:1 cultivar to breed true.
- Assuming CBD plants contain no THC.
- Confusing chemotype (the ratio) with potency (the amount).
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.CBGA (cannabigerolic acid) is the precursor cannabinoid that the plant converts into THCA, CBDA or CBCA, formed by joining olivetolic acid to geranyl pyrophosphate.THCA (tetrahydrocannabinolic acid) is the acid cannabinoid that drug-type cannabis makes in its trichomes; heat or time decarboxylates it into THC, losing about 12% of its mass.CBDA (cannabidiolic acid) is the acid cannabinoid made by CBDA synthase from CBGA; it dominates hemp-type and CBD-type cannabis and decarboxylates to CBD.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.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.F2 is the second filial generation, made by crossing or selfing F1 plants; it shows the widest segregation, bringing back hidden recessives and new combinations.Codominance is when both alleles at a locus show in a heterozygous plant, as with THCA and CBDA synthase, so a B_T/B_D plant makes both THC and CBD.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 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.Hemp is cannabis grown for fibre, seed or other uses and, in many laws, defined as Cannabis sativa with no more than 0.3% THC by dry weight.A seedling is a young cannabis plant from emergence of the cotyledons until it has several sets of true leaves, typically the first 2-3 weeks.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).CBG (cannabigerol) is the neutral, decarboxylated form of CBGA, the cannabinoid precursor; most plants convert nearly all of it onward, but some cultivars are bred to accumulate it.
