Locus (genetics)

  (Redirected from Genetic loci)
Parts of a typical chromosome:

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

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.


Cytogenetic banding nomenclature

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:

  • 3 = chromosome 3
  • p = p-arm
  • 22 = region 2, band 2 (read as "two, two", not "twenty-two")
  • 1 = sub-band 1

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.

Example of cytogenetic bands
Component Explanation
3 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 that q stands for queue, meaning "tail" in French).
22.1 The numbers that follow the letter represent the position on the arm: region 2, band 2, sub-band 1. 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 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.

See alsoEdit


  1. ^ Wood, E.J. (1995). "The encyclopedia of molecular biology". Biochemical .Education. 23 (2): 1165. doi:10.1016/0307-4412(95)90659-2.
  2. ^ Ezkurdia, Iakes; Juan, David; Rodriguez, Jose Manuel; Frankish, Adam; Diekhans, Mark; Harrow, Jennifer; Vazquez, Jesus; Valencia, Alfonso; Tress, Michael L. (2014-11-15). "Multiple evidence strands suggest that there may be as few as 19,000 human protein-coding genes". Human Molecular Genetics. 23 (22): 5866–5878. doi:10.1093/hmg/ddu309. ISSN 1460-2083. PMC 4204768. PMID 24939910.
  3. ^ "NCI Dictionary of Genetics". National Cancer Institute. Retrieved 13 December 2014.

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

External linksEdit