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In biology and evolutionary computation, a 'locus' (plural 'loci') is a fixed position on a chromosome, such as the position of a gene. 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 process of determining the locus for a particular biological trait.
Diploid and polyploid cells whose chromosomes have the same allele at some locus are called homozygous, while those that have different alleles at a locus, heterozygous.

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Nomenclature

External links

Nomenclature

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

'Component'	'Explanation'
''6''	The chromosome number.
''p''	The position is on the chromosome's short arm (''p'' for ''petit'' in French); ''q'' indicates the long arm.
''21.3''	The numbers following the letter represent the position on the arm: band 21, sub-band 3. The bands are visible under a microscope when the chromosome is suitably stained. Each of the bands is numbered, beginning with 1 for the band nearest the centromere. Sub-bands and sub-sub-bands are visible at higher resolution.

A range of locations 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 of sub-band 4 of band 1, and sub-band 1 of band 2.
The ends of a chromosome are labeled ''"ptel"'' and ''"qtel"'', and so ''"2qtel"'' refers to the telomere of the long arm of chromosome 2.

External links

★ Overview at ornl.gov