Ingenetics, alocus (pl.:loci) is a specific, fixed position on achromosome where a particulargene orgenetic marker is located.[1] Each chromosome carries many genes, with each gene occupying a different position or locus; in humans, the total number ofprotein-coding genes in a completehaploid set of 23 chromosomes is estimated at 19,000–20,000.[2]
Genes may possess multiple variants known asalleles, and an allele may also be said to reside at a particular locus.Diploid andpolyploid cells whose chromosomes have the same allele at a given locus are calledhomozygous with respect to that locus, while those that have different alleles at a given locus are calledheterozygous.[3] The ordered list of loci known for a particulargenome is called agene map.Gene mapping is the process of determining the specific locus or loci responsible for producing a particularphenotype orbiological trait.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.
The shorter arm of a chromosome is termed thep arm orp-arm, while the longer arm is theq arm orq-arm. The chromosomal locus of a typical gene, for example, might be written3p22.1, where:[citation needed]
Thus the entire locus of the example above would be read as "three P two two point one". Thecytogenetic bands are areas of the chromosome either rich in actively-transcribed DNA (euchromatin) or packaged DNA (heterochromatin). They appear differently upon staining (for example, euchromatin appears white and heterochromatin appears black onGiemsa staining). They are counted from thecentromere out toward thetelomeres.[citation needed]
| Component | Explanation |
|---|---|
| 3 | The chromosome number |
| p | The position is on the chromosome's short arm (a common apocryphal explanation is that thep stands forpetit in French);q indicates the long arm (chosen as next letter in alphabet after p; it is also said thatq stands forqueue, meaning "tail" in French[4]). |
| 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 amicroscope when thechromosome issuitably stained. Each of the bands are numbered, beginning with 1 for the band nearest thecentromere. Sub-bands and sub-sub-bands are visible at higher resolution.[citation needed] |
A range of loci is specified in a similar way. For example, the locus of geneOCA1 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.[citation needed]
The ends of a chromosome are labeled"pter" and"qter", and so"2qter" refers to the terminus of the long arm of chromosome 2.[citation needed]
