locus

Locus is a term from Latin meaning place. In many disciplines, it denotes a specific position or region, often relative to a reference object such as a chromosome, a geometric figure, or an anatomical structure. The plural form in biology and genetics is loci.

In mathematics and geometry, a locus is the set of all points that satisfy a given condition. Classic examples include the circle, which is the locus of points at a fixed distance from a fixed point, and the perpendicular bisector of a segment, which is the locus of points equidistant from its endpoints. Other loci define parabolas, ellipses, or more complex curves; in higher dimensions, loci may be surfaces or manifolds. The concept is central to describing constraints and shapes in coordinate form.

In genetics, a locus refers to the specific position of a gene or marker on a chromosome. An allele is a variant form of a locus. An individual’s genotype describes the pair of alleles at each locus, and combinations of loci underlie inheritance patterns, genetic mapping, and traits. Loci can be linked on chromosomes, influencing how traits are inherited, and modern studies often refer to many loci across the genome in association analyses.

In anatomy and general usage, locus can denote a particular site within a larger structure, as in named regions like the locus coeruleus in the brain. Across fields, locus conveys the idea of a defined place, whether on a coordinate plane, within a chromosome, or inside a biological system.