How do geneticists indicate the location of a gene?
Geneticists pinpoint a gene’s position on a chromosome using cytogenetic band patterns and precise molecular nucleotide coordinates.
Overview
Geneticists use maps to describe the location of a particular gene on a chromosome. One type of map uses the cytogenetic location, which is based on a distinctive pattern of bands created when chromosomes are stained with certain chemicals. Another type of map uses the molecular location, a precise description of a gene’s position derived from the DNA nucleotide sequence that makes up the chromosome.
Cytogenetic location
Geneticists employ a standardized notation to describe a gene’s cytogenetic location. In most cases the notation identifies a specific band on a stained chromosome, such as 17q12. When the exact position is less certain, a range of bands may be given, for example 17q12‑q21. The combination of numbers and letters forms a gene’s “address” and consists of several parts:
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Chromosome number or letter: The first element indicates the chromosome on which the gene resides. Autosomes are numbered 1 through 22; the sex chromosomes are designated X or Y.
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Chromosome arm: Each chromosome has a short arm (p) and a long arm (q) defined by the centromere. For example, 5q denotes the long arm of chromosome 5, while Xp denotes the short arm of the X chromosome.
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Band position: Position is expressed by two digits (region and band) and may include a decimal and additional digits for sub‑bands. Numbers increase with distance from the centromere. For instance, 14q21 is closer to the centromere than 14q22.
Abbreviations such as “cen” (centromere) and “ter” (terminus) are also used. “Cen” indicates proximity to the centromere (e.g., 16pcen), while “ter” marks the chromosome end (e.g., 14qter).
The CFTR gene is located on the long arm of chromosome 7 at position 7q31.2.
Molecular location
The Human Genome Project, completed in 2003, sequenced the nucleotides of each human chromosome. This sequence information enables researchers to assign a more specific address than the cytogenetic description. A gene’s molecular address pinpoints its exact nucleotide coordinates on a chromosome and indicates the gene’s size, allowing precise measurement of distances between genes.
Different research groups may report slightly different molecular coordinates because they interpret the genome sequence using varied methods, leading to small variations in a gene’s reported address.
Further information
For additional details on genetic mapping, see resources from the National Human Genome Research Institute (NHGRI) that explain how researchers create a genetic map.
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