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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. Each chromosome carries many genes, with each gene occupying a different position or locus; in humans, the total number of
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
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 ( euchromatin) or packaged DNA (
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.
Overview at ornl.gov
from NCBI. Genetics Chromosomes
protein-coding genes
The human genome is a complete set of nucleic acid sequences for humans, encoded as DNA within the 23 chromosome pairs in cell nuclei and in a small DNA molecule found within individual Mitochondrial DNA, mitochondria. These are usually treated s ...
in a complete haploid
Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...
set of 23 chromosomes is estimated at 19,000–20,000.
Genes may possess multiple variants known as alleles, and an allele may also be said to reside at a particular locus. Diploid
Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...
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. 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
A phenotypic trait, simply trait, or character state is a distinct variant of a phenotypic characteristic of an organism; it may be either inherited or determined environmentally, but typically occurs as a combination of the two.Lawrence, Eleano ...
. 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
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
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 ( euchromatin) or packaged DNA (heterochromatin
Heterochromatin is a tightly packed form of DNA or '' condensed DNA'', which comes in multiple varieties. These varieties lie on a continue between the two extremes of constitutive heterochromatin and facultative heterochromatin. Both play a role ...
). They appear differently upon staining (for example, euchromatin appears white and heterochromatin appears black on Giemsa staining
Giemsa stain (), named after German chemist and bacteriologist Gustav Giemsa, is a nucleic acid stain used in cytogenetics and for the histopathological diagnosis of malaria and other parasites.
Uses
It is specific for the phosphate groups of ...
). They are counted from the centromere out toward the telomeres.
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
* Chromosomal translocation *Cytogenetic notation
The following table summarizes symbols and abbreviations used in cytogenetics:
See also
*Chromosome abnormalities
*Directionality (molecular biology) for 3' and 5' notation
*locus (genetics) for basic notational system
*International System for ...
* Karyotype
A karyotype is the general appearance of the complete set of metaphase chromosomes in the cells of a species or in an individual organism, mainly including their sizes, numbers, and shapes. Karyotyping is the process by which a karyotype is disce ...
* Null allele
* International System for Human Cytogenetic Nomenclature The International System for Human Cytogenomic Nomenclature (previously International System for Human Cytogenetic Nomenclature), ISCN in short, is an international standard for human chromosome nomenclature, which includes band names, symbols and a ...
References
External links
{{Wikidata property , P4196Overview at ornl.gov
from NCBI. Genetics Chromosomes