Heterozygosity – Mixed Inheritance and Its Share in Cannabis

Heterozygosity means mixed inheritance: at a given place in the genome the plant carries two different variants rather than two copies of the same one. Such places are called loci, and the two variants at a locus are called alleles. In cannabis this is the starting state and not a defect, because the species normally pollinates across individuals rather than itself, so different variants keep meeting.

The practically important part comes next: reducing mixed inheritance brings more inconsistency before it brings less. That reverses what a round of selfing is expected to do, selfing being a round in which a plant is fertilised with its own pollen. In the comparison study the resulting offspring spread two to three times as far as offspring from an ordinary cross between two plants. Anyone reducing mixed inheritance has to sit through that phase, or the line gets abandoned exactly where it looks worst.

Where it shows and where it does not

The chemotype is the clear case, meaning the question of whether a plant makes mostly THC or mostly CBD. It hangs on a single place in the genome, the B-Locus. Every plant holds two of that place, and the two variants sitting there rank equally: neither masks the other. A plant carrying the same variant twice shows the respective pure chemotype; one carrying two different variants makes both compounds. That makes the outcome of a cross predictable here: two mixed parents yield 25 % pure THC plants, 50 % mixed ones and 25 % pure CBD plants. Across ten sets of second-generation offspring, with 35 to 118 plants scored per group, exactly that ratio came out.

Most other traits give no such signal. There one variant masks the other, and the hidden one surfaces only when the next generation segregates. A plant that looks flawless can be mixed at any number of loci, and the fact emerges a generation later.

How much of it the material carries

In open-pollinated source plants, meaning plants pollinated without control, the share of mixed loci ran between 17 and 35 %. That was measured against a panel of 1,500 markers – markers being places in the genome that can be read unambiguously in the laboratory and stand in for their surroundings. Deliberately inbred material is not free of it either; how far even repeatedly narrowed lines still segregate is covered in the article on homozygosity.

That determines the situation on the market. Most seed sold as hybrid comes from crosses between two individual, strongly mixed plants and yields correspondingly inconsistent offspring. Genuine first-generation hybrid seed, by contrast, comes from crossing two unrelated lines that are each true-breeding in themselves. For species that pollinate across individuals, the widespread expectation that a first daughter generation is uniform therefore turns around: selection begins there already, because the wanted combinations can appear in that very generation.

The two routes to reducing it

Two routes lead there. One is sib-mating, the pairing of two plants from the same progeny; it needs male plants and therefore regular seed. The other is selfing, which in cannabis requires induced sex reversion, because male and female flowers usually sit on separate plants.

Selfing is the faster of the two: it halves the share of mixed loci per round, while sib-mating takes longer. That pace carries a cost the time calculation does not show. At equal levels of inbreeding, sib-mating clears out the variants that are lethal in double copy more effectively, 7.11 remaining variants in the model against 9.3 under selfing, because homozygosity rises step by step rather than in one jump. The model describes natural populations; what transfers is the mechanism, not the number. On top of that, every round of selfing also selects for the capacity to undergo sex reversion, which raises the risk of unwanted male flowers in the finished line.

What reducing it costs

A single round of selfing lowers mixed inheritance measurably and hits the plants at the same time. Compared directly against two ordinary crosses, the expected share of mixed loci in the selfed offspring stood at 0.085 against roughly 0.10 in the crosses, about a sixth lower. The same plants reached about half the height and half the leaf area, produced 63 % less floral dry weight than the comparison group and showed variegated leaves throughout.

The spread is what matters: in total and floral mass it came out two to three times as large as in the crosses. The round did not unify, it exposed what had been masked. A progeny becomes consistent only at the end of a long inbreeding series, and then in the cross between two such lines.

How it is measured

The standard route is a marker panel reporting the share of mixed places across many loci. For the chemotype an additional marker exists that shows both variants at once: it identified the mixed plants in 95.3 % of cases, missing roughly one in twenty. That figure was obtained on the lines and progenies of a single study, and the authors had no data on the marker’s value beyond those crosses.