A chromosome map indicates the relative positions of the genes that code for certain characteristics. The basic format for writing a gene position is the chromosome number, arm, band, sub-band, and sub-sub-band, if known. An example is 3p22.5.
The chromosome number refers to one of the 22 autosomal chromosomes (numbered 1–22) or one of the two sex-determining chromosomes, X and Y. In the example, the gene is on chromosome 3.
Each chromosome has two arms, which are separated by a centromere, the pinched-in area at or above the middle of the chromosome. The short arm, labeled p, is above the centromere, and the long arm, q, is below it. In the case of the example gene, it is found on the short arm (p) of chromosome 3, or 3p.
The arms are further divided into cytogenetic bands (regions) numbered 1, 2, 3, etc. The numbers start at the centromere and increase toward the end of the arm, known as the telomere. These bands can only be seen when stained and viewed under a microscope. Sub-bands, which are numbered the same way as bands, may be visible within bands at greater magnifications. Therefore, the exact location of the example gene is the short arm (p) of chromosome 3, band 2, sub-band region 2, and a subsub band 5.
The following 24 illustrations demonstrate the approximate gene location for several of the genes relating to disorders mentioned in this encyclopedia. Disorders known to be related to a specific chromosome but not necessarily at an exact location have been placed below the chromosome. These chromosome maps are in no way complete; rather, they provide an introduction to understanding relative size differences of human chromosomes and where geneticists have located the genes associated with the source of certain genetic disorders.