A QTL (quantitative trait locus) is a stretch of the genome whose variants are associated with differences in a quantitative trait. A QTL may contain one causal gene or several; mapping often narrows the location long before anyone knows which gene is responsible.
In practice
QTL are found by growing a segregating population (an F2 or similar), measuring the trait, genotyping each plant at many markers, and testing which markers track the trait. Genome-wide association studies (GWAS) do the same across unrelated plants. In cannabis, studies have mapped regions for flowering time and sex expression in hemp (Petit et al. 2020) and major flowering-time loci such as Autoflower1 and Early1 (Toth et al. 2022).
For breeders the payoff is marker-assisted selection: screen seedlings for the markers and keep only those likely to carry the good allele, before flowering.
Don't confuse with
- Gene: a QTL is a region; the gene inside it may still be unknown.
- Major gene: a QTL with a large effect (like Autoflower1) behaves almost like a Mendelian gene; most QTL are small.
- Proof of a trait: a QTL shifts probabilities. Plants carrying the good allele still vary.
The stretch is the rapid increase in height in the first two to three weeks after switching to flower, when many cultivars grow 1.5 to 3 times taller.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 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.Segregation is the separation of a plant's two alleles at each locus into different gametes, so offspring of a heterozygous parent vary in which allele they get.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.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.Marker-assisted selection is choosing seedlings or parents by testing DNA markers linked to wanted traits, so plants can be selected before they flower.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 locus is the fixed position on a chromosome where a particular gene or DNA marker sits; the plural is loci.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.Linkage is the tendency of genes that sit close together on the same chromosome to be inherited together, rather than independently.
