Locus (genetics)

In the fields of genetics and genetic computation, a locus (plural loci) is the specific location of a gene or DNA sequence on a chromosome. A variant of the DNA sequence at a given locus is called an allele. The ordered list of loci known for a particular genome is called a genetic map. Gene mapping is the procession of determining the locus for a particular biological trait.

Diploid and polyploid cells whose chromosomes have the same allele of a given gene at some locus are called homozygous with respect to that gene, while those that have different alleles of a given gene at a locus, heterozygous with respect to that gene.

Nomenclature
The chromosomal locus of a gene might be written "6p21.3".

A range of locales 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, some where in the range from sub-band 4 of band 1, and sub-band 1 of band 2.

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

Centisome
A centisome is defined as 1% of a chromosome length.