How do geneticists indicate the location of a gene?

Geneticists use a standardized way of describing the location of a particular gene on a chromosome. A gene's location is often written as a position:

17q12

It can also be written as a range, if less is known about the exact location:

17q12-q21

These combinations of numbers and letters provide a gene's “address” on a chromosome. The address is made up of several parts:

• The chromosome on which the gene can be found. The first number or letter used to describe a gene's location represents the chromosome. Chromosomes 1 through 22 (the autosomes) are designated by their chromosome number. The sex chromosomes are designated by X or Y.
• The arm of the chromosome. Each chromosome is divided into two sections, or arms, based on the location of a narrowing (constriction) called the centromere. By convention, the shorter arm is called p, and the longer arm is called q. The chromosome arm is the second part of the gene's address. For example, 5q is the long arm of chromosome 5, and Xp is the short arm of the X chromosome.
• The position of the gene on the p or q arm. The position of a gene is based on a standard pattern of light and dark bands that appear when the chromosome is stained in a certain way. The position is usually designated by two digits (representing a region and a band), which are sometimes followed by a decimal point and one or more additional digits (representing sub-bands within a light or dark area). The number indicating gene position increases with distance from the centromere. For example: 14q21 represents the long arm of chromosome 14 at position 21. 14q21 is closer to the centromere than 14q22.

Sometimes, the abbreviations “cen” or “ter” are also used to describe a gene's location. “Cen” indicates that the gene is very close to the centromere. For example, 16pcen refers to the short arm of chromosome 16 near the centromere. “Ter” stands for terminus, which indicates that the gene is very close to the end of the p or q arm. For example, 14qter refers to tip of the long arm of chromosome 14. (“Tel” is also sometimes used to describe a gene's location. “Tel” stands for telomeres, which are at the ends of each chromosome. The abbreviations “tel” and “ter” refer to the same location.)

The CFTR gene is located on the long arm of chromosome 7 at position 7q31.2.

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