A locus, as related to genomics, is a physical site or location within a genome (such as a gene or another DNA segment of interest), somewhat like a street address. The plural of locus is loci.
Locus. Locus is a term that we use to tell us where on a chromosome a specific gene is. So it's really the physical location of a gene on a chromosome. It's a way of defining the gene's neighborhood. If you consider the entire chromosome as a country where the gene is found, and then a region of the chromosome would be the city. The more specific area, or the locus, is this particular neighborhood where the gene is found.
Ellen Sidransky, M.D.
Chief and Senior Investigator
Medical Genetics Branch