genetics Genetics is the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) It is an important branch in biology because heredity is vital to organisms' evolution. Gregor Mendel, a Moravian Augustinian friar work ...

, a locus (plural loci) is a specific, fixed position on a

chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins ar ...

where a particular

gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...

genetic marker A genetic marker is a gene or DNA sequence with a known location on a chromosome that can be used to identify individuals or species. It can be described as a variation (which may arise due to mutation or alteration in the genomic loci) that can be ...

is located. 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. Genes may possess multiple variants known as

allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chro ...

s, and an allele may also be said to reside at a particular locus. Diploid and

polyploid Polyploidy is a condition in which the cells of an organism have more than one pair of ( homologous) chromosomes. Most species whose cells have nuclei ( eukaryotes) are diploid, meaning they have two sets of chromosomes, where each set contain ...

cells whose chromosomes have the same allele at a given locus are called

homozygous Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mo ...

with respect to that locus, while those that have different alleles at a given locus are called

heterozygous Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mo ...

. The ordered list of loci known for a particular

genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding g ...

is called a gene map.

Gene mapping Gene mapping describes the methods used to identify the locus of a gene and the distances between genes. Gene mapping can also describe the distances between different sites within a gene. The essence of all genome mapping is to place a c ...

is the process of determining the specific locus or loci responsible for producing a particular

phenotype In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological pr ...

or biological trait.

Association mapping In genetics, 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 geno ...

, 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 Chromosome 3 is one of the 23 pairs of chromosomes in humans. People normally have two copies of this chromosome. Chromosome 3 spans almost 200 million base pairs (the building material of DNA) and represents about 6.5 percent of the total DNA in ...

* ''p'' = p-arm * ''22'' = region 2,

band Band or BAND may refer to: Places *Bánd, a village in Hungary *Band, Iran, a village in Urmia County, West Azerbaijan Province, Iran * Band, Mureș, a commune in Romania *Band-e Majid Khan, a village in Bukan County, West Azerbaijan Province, I ...

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 Euchromatin (also called "open chromatin") is a lightly packed form of chromatin ( DNA, RNA, and protein) that is enriched in genes, and is often (but not always) under active transcription. Euchromatin stands in contrast to heterochromatin, whi ...

) or packaged DNA ( heterochromatin). They appear differently upon staining (for example, euchromatin appears white and heterochromatin appears black on Giemsa staining). They are counted from the

centromere The centromere links a pair of sister chromatids together during cell division. This constricted region of chromosome connects the sister chromatids, creating a short arm (p) and a long arm (q) on the chromatids. During mitosis, spindle fibers ...

out toward the telomeres. Human karyotype with bands and sub-bands

A range of loci is specified in a similar way. For example, the locus of gene

OCA1 Oculocutaneous albinism type I or type 1A is an autosomal recessive skin disease. This subtype of oculocutaneous albinism is caused when the gene for tyrosinase (symbol TYR or OCA1) does not function properly. The location of OCA1 may be written ...

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.

References

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

External links

{{Wikidata property , P4196
Overview at ornl.gov

from NCBI. Genetics Chromosomes

Nomenclature

See also

References

External links