A genetic locus (plural: loci) is a fixed position on a chromosome carrying a particular gene or other DNA sequence. A variant of a DNA sequence at a particular locus is called an allele for that locus. Gene mapping, by techniques such as the three-point test cross, enable us to order and map the distance between loci that are linked on a particular chromosome. Homozygosity is the property of having identical alleles at homologous loci, while heterozygosity is the property of having different alleles.

The nomenclature for chromosomal loci is best described by example, e.g. the locus 8p22.4 means:

8 denotes that the locus is on chromosome 8
p denotes that the locus is on the short arm (p), as opposed to the long arm (q)
22.4 denotes the specific position of the locus within the banding of the short arm, as identified by staining: band 2, section 2, sub-band 4

A range of loci using this nomenclature (for instance 8p22.4-23.4) may also be used when talking about, for example, a specific haplotype that co-segregates with a disease state.