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Locus (genetics)

Parts of a typical chromosome:(1) Chromatid (2) Centromere (3) Short (p) arm(4) Long (q) arm In genetics , a locus ( plural "}]],"parts":[{"template":{"target":{"wt":"plural for...

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Parts of a typical chromosome:(1) Chromatid(2) Centromere(3) Short (p) arm(4) Long (q) arm

In genetics, a locus (pl.: 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. 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

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

Así, el locus completo del ejemplo anterior se leería como "tres P dos dos coma uno". Las bandas citogenéticas son áreas del cromosoma ricas en ADN activamente transcrito ( eucromatina ) o en ADN empaquetado ( heterocromatina ). Se ven diferentes al teñirlas (por ejemplo, la eucromatina aparece blanca y la heterocromatina negra con la tinción de Giemsa ). Se cuentan desde el centrómero hacia los telómeros .

Ejemplo de bandas citogenéticas

De forma similar, se especifica un rango de loci. Por ejemplo, el locus del gen OCA1 puede escribirse como "11q1.4-q2.1", lo que significa que se encuentra en el brazo largo del cromosoma 11, en algún punto del rango comprendido entre la subbanda 4 de la región 1 y la subbanda 1 de la región 2.

Los extremos de un cromosoma se denominan "pter" y "qter" , por lo que "2qter" se refiere al extremo del brazo largo del cromosoma 2.

Véase también

Referencias

  1. ↑ Wood, EJ (1995). "La enciclopedia de biología molecular". 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. Archived from the original on 26 April 2015. Retrieved 13 December 2014.
  4. ↑"NCBI Genetics Review". National Center for Biotechnology Information. Archived from the original on 9 May 2023. Retrieved 10 March 2021.
  • Michael, R. Cummings. (2011). Human Heredity. Belmont, California: Brooks/Cole.
  • Overview at ornl.gov
  • Chromosome Banding and Nomenclature from NCBI.