In genetics, a locus (plural loci) is a specific, fixed position on a chromosome where a particular gene or genetic marker is located.^[1] Each chromosome carries many genes, with each gene occupying a different position or locus; in humans, the total number of protein-coding genes in a complete haploid set of 23 chromosomes is estimated at 19,000–20,000.^[2]

Genes may possess multiple variants known as alleles, and an allele may also be said to reside at a particular locus. Diploid and polyploid cells whose chromosomes have the same allele at a given locus are called homozygous with respect to that locus, while those that have different alleles at a given locus are called heterozygous.^[3] The ordered list of loci known for a particular genome is called a gene map. Gene mapping is the process of determining the specific locus or loci responsible for producing a particular phenotype or biological trait.

The shorter arm of a chromosome is termed the p arm or p-arm, while the longer arm is the q arm or q-arm. The chromosomal locus of a typical gene, for example, might be written 3p22.1, where:

Thus the entire locus of the example above would be read as "three P two two point one". The cytogenetic bands are counted from the centromere out toward the telomeres.

A range of loci is specified in a similar way. For example, the locus of gene OCA1 may be written "11q1.4-q2.1", meaning it is on the long arm of chromosome 11, somewhere in the range from sub-band 4 of region 1 to sub-band 1 of region 2.

The ends of a chromosome are labeled "pter" and "qter", and so "2qter" refers to the terminus of the long arm of chromosome 2.