View source for Biology:Locus (genetics)

{{short description|Location of a gene or region on a chromosome}}
[[Image:Chromosome.svg|thumb|Parts of a typical chromosome:<br><br>(1) [[Biology:Chromatid|Chromatid]]<br>(2) Centromere<br>(3) Short (p) arm<br>(4) Long (q) arm]]
In [[Biology:Genetics|genetics]], a '''locus''' ({{plural form}}: '''loci''') is a specific, fixed position on a [[Biology:Chromosome|chromosome]] where a particular [[Biology:Gene|gene]] or [[Biology:Genetic marker|genetic marker]] is located.<ref>{{cite journal|doi=10.1016/0307-4412(95)90659-2|title=The encyclopedia of molecular biology|journal= Biochemical Education|volume=23|issue=2|pages=1165|year=1995|last1=Wood|first1=E.J.}}</ref> Each chromosome carries many genes, with each gene occupying a different position or locus; in humans, the total number of [[Biology:Human genome#Coding sequences (protein-coding genes)|protein-coding genes]] in a complete [[Biology:Haploid|haploid]] set of 23 chromosomes is estimated at 19,000–20,000.<ref>{{Cite journal|last1=Ezkurdia|first1=Iakes|last2=Juan|first2=David|last3=Rodriguez|first3=Jose Manuel|last4=Frankish|first4=Adam|last5=Diekhans|first5=Mark|last6=Harrow|first6=Jennifer|last7=Vazquez|first7=Jesus|last8=Valencia|first8=Alfonso|last9=Tress|first9=Michael L.|date=2014-11-15|title=Multiple evidence strands suggest that there may be as few as 19,000 human protein-coding genes|journal=Human Molecular Genetics|volume=23|issue=22|pages=5866–5878|doi=10.1093/hmg/ddu309|issn=1460-2083|pmc=4204768|pmid=24939910}}</ref>

Genes may possess multiple variants known as [[Biology:Allele|allele]]s, 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.<ref>{{cite web|title=NCI Dictionary of Genetics|url=http://www.cancer.gov/geneticsdictionary?expand=H|publisher=[[Organization:National Cancer Institute|National Cancer Institute]]|access-date=13 December 2014|archive-date=26 April 2015|archive-url=https://web.archive.org/web/20150426162238/http://www.cancer.gov/geneticsdictionary?expand=H|url-status=live}}</ref> The ordered list of loci known for a particular [[Biology:Genome|genome]] is called a [[Biology:Gene map|gene map]]. [[Biology:Gene mapping|Gene mapping]] is the process of determining the specific locus or loci responsible for producing a particular [[Biology:Phenotype|phenotype]] or biological trait. [[Organization:Association mapping|Association mapping]], also known as "linkage disequilibrium mapping", is a method of mapping quantitative trait loci (QTLs) that takes advantage of historic linkage disequilibrium to link phenotypes (observable characteristics) to genotypes (the genetic constitution of organisms), uncovering genetic associations.

== Nomenclature ==
[[File:Cytogenetic Banding Nomenclature.png|thumb|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, [[Biology:Karyotype#Types of banding|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 areas of the chromosome either rich in actively-transcribed DNA ([[Biology:Euchromatin|euchromatin]]) or packaged DNA ([[Biology:Heterochromatin|heterochromatin]]). They appear differently upon staining (for example, euchromatin appears white and heterochromatin appears black on Giemsa staining). They are counted from the [[Biology:Centromere|centromere]] out toward the [[Biology:Telomere|telomere]]s.
[[Image:Neurofibromatosis2-locus.svg|thumb|50px|Example of cytogenetic bands]]
[[File:Human karyotype with bands and sub-bands.png|thumb|Human [[Biology:Karyotype|karyotype]] with annotated bands and sub-bands. It shows 22 homologous autosomal chromosome pairs, both the female (XX) and male (XY) versions of the two sex chromosomes, as well as the mitochondrial genome (at bottom left). {{further|Karyotype}}]]
{| class="wikitable"
| '''Component''' || '''Explanation''' 
 |- 1.2
 | ''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; it is also said that ''q'' stands for ''queue'', meaning "tail" in French<ref>{{cite web|title=NCBI Genetics Review|url=https://www.ncbi.nlm.nih.gov/Class/MLACourse/Original8Hour/Genetics/chrombanding.html|publisher=[[Organization:National Center for Biotechnology Information|National Center for Biotechnology Information]]|access-date=10 March 2021|archive-date=9 May 2023|archive-url=https://web.archive.org/web/20230509060227/https://www.ncbi.nlm.nih.gov/Class/MLACourse/Original8Hour/Genetics/chrombanding.html|url-status=live}}</ref>).
 |- 5.6
 | ''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 [[Engineering:Microscope|microscope]] when the [[Biology:Chromosome|chromosome]] is [[Biology:G banding|suitably stained]]. Each of the bands is numbered, beginning with 1 for the band nearest the [[Biology:Centromere|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 also ==
* [[Biology:Chromosomal translocation|Chromosomal translocation]]
* [[Biology:Cytogenetic notation|Cytogenetic notation]]
* [[Biology:Karyotype|Karyotype]]
* [[Biology:Null allele|Null allele]]
* [[Biology:International System for Human Cytogenetic Nomenclature|International System for Human Cytogenetic Nomenclature]]

== References ==
<references/>
Michael, R. Cummings. (2011). ''Human Heredity''. Belmont, California: Brooks/Cole.

== External links ==
{{Wikidata property |P4196 }} 
* [http://www.ornl.gov/sci/techresources/Human_Genome/posters/chromosome/omim.shtml#GeneMap Overview at ornl.gov]
* [https://www.ncbi.nlm.nih.gov/Class/MLACourse/Original8Hour/Genetics/chrombanding.html Chromosome Banding and Nomenclature] from NCBI.
[[Category:Genetics]]
[[Category:Chromosomes]]

{{Sourceattribution|Locus (genetics)}}