Locus (genetics)

Chromosome components:

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

A locus (plural loci) in genetics is a fixed position on a chromosome, like the position of a gene or a marker (genetic marker).[1] Each chromosome carries many genes; human's estimated 'haploid' protein coding genes are 19,000–20,000,[2] 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.[3]

Nomenclature

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 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 count from the centromere out toward the telomeres.

ComponentExplanation
3The chromosome number.
pThe 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).
22.1The 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 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.
Example of bands

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.

Centisome

A centisome (not to be confused with a centrosome) is defined as 1% of a chromosome length.[4]

See also

References

  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.
  4. ↑ Peter D. Karp; Monica Riley (2009-01-11), Representations of Metabolic Knowledge (PDF)

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

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