A locus is the fixed physical position on a chromosome where a gene or a DNA marker sits. The plural is loci. Every plant of the species has the same loci in the same order (barring rearrangements); what differs between plants is which alleles occupy them.
In practice
Breeders talk about loci when one position controls a clear trait. The cannabinoid synthase region is often called the B locus, after de Meijer's model of THC/CBD inheritance. Autoflowering behaviour maps to a major locus named Autoflower1 (Toth et al. 2022). DNA tests report results locus by locus: a genotyping panel might read a few thousand SNP loci and compare them between two cuts.
When a trait is controlled by many loci of small effect, the regions are called quantitative trait loci (QTL). Most yield and vigour traits fall in that group.
Don't confuse with
- Gene: a gene occupies a locus, but a locus can also be a marker with no known function.
- Allele: the variant found at the locus.
- Chromosome: the whole DNA molecule that carries thousands of loci. Cannabis has 10 chromosome pairs.
A chromosome is one long DNA molecule packaged with proteins that carries many genes; cannabis normally has 10 pairs, nine autosome pairs plus an X/Y sex pair.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.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 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.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.A day-neutral cannabis plant that starts flowering by age, usually a few weeks after germination, regardless of light schedule; the trait is mainly recessive.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 QTL (quantitative trait locus) is a region of the genome statistically linked to variation in a quantitative trait such as yield or flowering time.Linkage is the tendency of genes that sit close together on the same chromosome to be inherited together, rather than independently.
