Locus (genetics)

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In genetics, a locus (plural loci) is a specific, fixed position on a chromosome where a particular gene or genetic marker is located. Locus (genetics)_sentence_0

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. Locus (genetics)_sentence_1

Genes may possess multiple variants known as alleles, and an allele may also be said to reside at a particular locus. Locus (genetics)_sentence_2

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. Locus (genetics)_sentence_3

The ordered list of loci known for a particular genome is called a gene map. Locus (genetics)_sentence_4

Gene mapping is the process of determining the specific locus or loci responsible for producing a particular phenotype or biological trait. Locus (genetics)_sentence_5

Nomenclature Locus (genetics)_section_0

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. Locus (genetics)_sentence_6

The chromosomal locus of a typical gene, for example, might be written 3p22.1, where: Locus (genetics)_sentence_7

Locus (genetics)_unordered_list_0

  • 3 = chromosome 3Locus (genetics)_item_0_0
  • p = p-armLocus (genetics)_item_0_1
  • 22 = region 2, band 2 (read as "two, two", not "twenty-two")Locus (genetics)_item_0_2
  • 1 = sub-band 1Locus (genetics)_item_0_3

Thus the entire locus of the example above would be read as "three P two two point one". Locus (genetics)_sentence_8

The cytogenetic bands are areas of the chromosome either rich in actively-transcribed DNA (euchromatin) or packaged DNA (heterochromatin). Locus (genetics)_sentence_9

They appear differently upon staining (for example, euchromatin appears white and heterochromatin appears black on Giemsa staining). Locus (genetics)_sentence_10

They are counted from the centromere out toward the telomeres. Locus (genetics)_sentence_11

Locus (genetics)_table_general_0

ComponentLocus (genetics)_cell_0_0_0 ExplanationLocus (genetics)_cell_0_0_1
3Locus (genetics)_cell_0_1_0 The chromosome numberLocus (genetics)_cell_0_1_1
pLocus (genetics)_cell_0_2_0 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).Locus (genetics)_cell_0_2_1
22.1Locus (genetics)_cell_0_3_0 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.Locus (genetics)_cell_0_3_1

A range of loci is specified in a similar way. Locus (genetics)_sentence_12

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. Locus (genetics)_sentence_13

The ends of a chromosome are labeled "pter" and "qter", and so "2qter" refers to the terminus of the long arm of chromosome 2. Locus (genetics)_sentence_14

See also Locus (genetics)_section_1

Locus (genetics)_unordered_list_1

Credits to the contents of this page go to the authors of the corresponding Wikipedia page: (genetics).