A chromosome map describes the order of genes and other markers, and the spacing between them on the chromosome. These maps are constructed with different levels of characterization and resolution. Physical mapping of a chromosome is obtained by determining the specific physical location of its genes and markers. This kind of mapping is an important process when searching for disease genes by positional cloning methodology and for DNA sequencing. It is to be distinguished from genetic mapping, also known as linkage mapping, which shows the position of genes and markers relative to each other rather than as physical landmarks on the chromosome.

The distances between markers on a genetic map are statistical estimates made by determining how frequently the two markers are passed together from parent to child. These distances are given in centiMorgan (cM), a unit related to the frequency of...