locus

A locus is the address of a gene — the specific, fixed place on a particular chromosome where it lives. Just as a building stays at one street address even though different tenants may move in and out, a gene's locus stays put even though different versions of the gene may occupy it.

Because a locus is a position rather than a particular sequence, the same locus on a pair of homologous chromosomes can carry different alleles — different variants of the gene. Describing where a locus sits uses a standard notation based on the chromosome's banding pattern: for example, the cystic fibrosis gene is located at 7q31, meaning chromosome 7, long arm (q), region 31.

Loci are the coordinates of genetics. Geneticists map them to track how genes are inherited, to measure how close two genes lie on a chromosome (closely linked loci tend to be passed on together), and to pinpoint the locations of disease-associated variants. The plural is loci, and the term is not limited to genes — any defined landmark on the genome, such as a single-letter marker, can be called a locus.