Marker-assisted selection (MAS) uses DNA markers tightly linked to a trait to decide which plants to keep, rather than waiting for the trait to show. A leaf punch from a seedling is tested, and only plants carrying the wanted marker alleles are grown on. Genomic selection extends the idea to many markers at once for quantitative traits.
In practice
The most useful cannabis markers so far are for traits with simple genetics: sex (a Y-linked marker), cannabinoid synthase type, and major flowering-time loci such as Autoflower1 (Toth et al. 2020; Toth et al. 2022). Screening 200 seedlings for sex and chemotype in week two saves months of space compared with growing them all to flower.
MAS is weaker for complex traits like yield, terpene profile or total cannabinoids, where many genes each contribute a little and the environment adds noise. It also depends on linkage: if the marker is not inside the gene, recombination can break the association in some offspring.
Don't confuse with
- Genetic engineering: MAS selects among natural variation; it does not change DNA.
- Genotyping for identity: the same tools, used to compare plants rather than to select.
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.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 quantitative trait varies continuously (yield, height, flowering time, cannabinoid content) because it is controlled by many genes of small effect plus the environment.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 terpene profile is the measured mix of terpenes in a sample, reported as each compound's % w/w and the total, used to describe and compare cultivar aroma chemistry.Total cannabinoids is the sum of all cannabinoids a lab measured in a sample, usually reported in % w/w, with acid forms either summed raw or decarboxylation-adjusted.Linkage is the tendency of genes that sit close together on the same chromosome to be inherited together, rather than independently.Genotyping is reading a plant's DNA at selected markers to identify it, compare it with other plants, check sex or chemotype, or estimate its ancestry.A SNP (single nucleotide polymorphism) is a position in the DNA where plants differ by one base letter; thousands of them form the markers used in cannabis genotyping.A QTL (quantitative trait locus) is a region of the genome statistically linked to variation in a quantitative trait such as yield or flowering time.Selection in breeding is choosing which plants become parents, so that alleles behind the chosen traits become more common in the next generation.
