A locus is the physical location on a chromosome where a gene, allele, or DNA sequence is present1. The term locus translates from Latin as either “place” or “location”. The plural form of the term is loci. In diploid organisms, the same locus is present twice within the nucleus of a cell (i.e. once on each chromosome). An individual can be classified as either homozygous or heterozygous at a specific locus if the two alleles present at that locus are the same or different, respectively.
Each locus must have a marker which allows it to be identified and mapped. A marker could be any number of things, but must be able to identify the locus specifically on a genetic map2. To identify a specific locus, specific nomenclature tools are used. These often vary between disciplines and study organisms and use various terms and symbols to create terms for each locus that are unique within that organism. In general, these terms often include references to which chromosome the locus is present on, which arm of the chromosome it is on, and the position within an individual band, or it may refer to a general location near a specific gene3.
1. Genetics Home Reference. Published March 16, 2015. Locus. U.S. National Library of Medicine. Accessed March 17, 2015 from http://ghr.nlm.nih.gov/glossary=locus.
2. Eppig, J.T., and M. Shimoyama. 2014. Guidelines for nomenclature of genes, genetic markers, alleles, and mutations in mouse and rat. International Committee on Standardized Genetic Nomenclature for Mice. Accessed March 17, 2015 from http://www.informatics.jax.org/mgihome/nomen/gene.shtml#trdef.
3. Wain, H.M., E.A. Bruford, R.C. Lovering, M.J. Lush, M.W. Wright, and S. Povey. 2002. Guidelines for human gene nomenclature. Genomics 79:464-470.