Chromosome ideograms: Understanding the ISCN mapping system

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A consistent numbering system is essential for mapping chromosomes. In Paris 1971 a mapping system known as the International System for Cytogenetic Nomenclature (ISCN) was established. 

The rules of the ISCN numbering system

1. Numbering of a chromosome begins at its centromere. 
2. Chromosomes are assigned a long arm and a short arm, based on the position of their centromere. The shorter arm of the chromosome is known as the p, or petite arm, from the French word for "small." The longer arm is known as the q. Chromosomal regions that are present on the short arm will begin with the designation p, while regions on the long arm will begin with q. 
3. By convention, the p arm of the chromosome is always shown at the top in a karyotype. 
4. Each arm of the chromosome is divided into regions. The numbers assigned to each region gets larger as the distance from the centromere to the telomere increases. Regions are identified by specific morphological features that are consistently found on a chromosome, such as the presence of a prominent Giemsa-staining band. The regions are named p1, p2, etc., on the short arm and q1, q2, etc., on the long arm. 
5. Depending on the resolution of the staining procedure, it may also be possible to detect additional bands within each region, which are designated by adding a digit to the number of the region, increasing in value as the distance from the centromere increases.

Ideogram of the medium-sized sub-metacentric chromosome 12 (400)

The position of the centromere separates the p and q arms (hatched area). This ideogram describes the pattern of Giemsa staining at a fairly low resolution (about 400 total number of bands in a karyotype). At this resolution, the long q arm of chromosome 12 is subdivided into two main regions: 12q1 and 12q2. Region 12q1 is further subdivided into five sub-regions: 12q11 through 12q15, each of which corresponds to a band detected by Giemsa staining. We refer to these subdivisions as "12q one-one" through "12q one-five" (not as "12q eleven" through "12q fifteen"). The more distal 12q2 region is subdivided into sub-regions 12q21 through 12q24. Sub-region 12q24 is further subdivided into regions 12q24.1 through 12q24.3.

Ideogram of the medium-sized sub-metacentric chromosome 12 at higher resolution

When chromosomes that are in prometaphase are stained a higher resolution is obtained, because prometaphase chromosomes are slightly less condensed than metaphase chromosomes. At this higher level of resolution approximately 850 bands are distinguishable in a karyotype.