Locus (genetics)

Locus (genetics)

Short and long arms
Chromosome components:

(1) Chromatid
(2) Centromere
(3) Short (p) arm
(4) Long (q) arm
Example of bands

A locus (plural loci) is the specific location or position of a gene, DNA sequence, on a chromosome, in the field of genetics. Each chromosome carries many genes; humans' estimated 'haploid' protein coding genes are 20,000-25,000, on the 23 different chromosomes. A variant of the similar DNA sequence located at a given locus is called an allele. The ordered list of loci known for a particular genome is called a gene 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 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, are called heterozygous with respect to that gene.[1]


  • Nomenclature 1
  • Centisome 2
  • See also 3
  • References 4
  • External links 5


The chromosomal locus of a gene might be written "6p21.3". Because "21" refers to "region 2, band 1" this is read as "two one", not as "twenty-one". So the entire locus is "six P two one point three."

Component Explanation
6 The chromosome number.
p The position is on the chromosome's short arm (a common apocryphal explanation is that the p stands for petit in French); q indicates the long arm (chosen as next letter in alphabet after p; alternatively it is sometimes said q is short for queue meaning tail in French).
21.3 The numbers that follow the letter represent the position on the arm: region 2, band 1, sub-band 3. The bands are visible under a microscope when 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 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.


A centisome (not to be confused with a centrosome) is defined as 1% of a chromosome length.[2]

See also


  1. ^ "NCI Dictionary of Genetics".  
  2. ^ Peter D. Karp, Monica Riley (2009-01-11), Representations of Metabolic Knowledge (PDF) 

Michael, R. Cummings. (2011). Human Heredity. Belmont, CA: Brooks/Cole

External links

  • Overview at
  • Chromosome Banding and Nomenclature from NCBI.