Locus (genetics)
In the fields of genetics and genetic computation, a locus (plural loci) is the specific location of a gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

 or DNA sequence
DNA sequence
The sequence or primary structure of a nucleic acid is the composition of atoms that make up the nucleic acid and the chemical bonds that bond those atoms. Because nucleic acids, such as DNA and RNA, are unbranched polymers, this specification is equivalent to specifying the sequence of...

 on a chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...

. A variant of the DNA sequence at a given locus is called an allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...

. The ordered list of loci known for a particular genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....

 is called a genetic map. Gene mapping
Gene mapping
Gene mapping, also called genome mapping, is the creation of a genetic map assigning DNA fragments to chromosomes.When a genome is first investigated, this map is nonexistent. The map improves with the scientific progress and is perfect when the genomic DNA sequencing of the species has been...

 is the procession 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.


The chromosomal locus of a gene might be written "6p21.3".
Component Explanation
>- 1.2
| 6
| p
p for petit in French); q indicates the long arm (chosen as next letter in alphabet after p).
>- 5.6
| 21.3
The numbers that follow the letter represent the position on the arm: region 2, band 1, sub-band 3. The bands are visible under a microscope
A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...

 when the chromosome is suitably stained
G banding
G-banding is a technique used in cytogenetics to produce a visible karyotype by staining condensed chromosomes.It is useful for identifying various genetic diseases through the photographic representation of the entire chromosome complement. The metaphase chromosomes are treated with trypsin and...

. Each of the bands is numbered, beginning with 1 for the band nearest the centromere
A centromere is a region of DNA typically found near the middle of a chromosome where two identical sister chromatids come closest in contact. It is involved in cell division as the point of mitotic spindle attachment...

. Sub-bands and sub-sub-bands are visible at higher resolution.

A range of locales 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 associated with albinism. This type of albinism is caused when the gene OCA1 does not function properly....

 may be written "11q1.4-q2.1", meaning it is on the long arm of chromosome 11, some where in the range from sub-band 4 of band 1, and sub-band 1 of band 2.

The ends of a chromosome are labeled "pter" and "qter", and so "2qter" refers to the telomere
A telomere is a region of repetitive DNA sequences at the end of a chromosome, which protects the end of the chromosome from deterioration or from fusion with neighboring chromosomes. Its name is derived from the Greek nouns telos "end" and merοs "part"...

of the long arm of chromosome 2.

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