Centimorgans

Centimorgan (cM) is a unit of genetic distance used to express the relative positions of loci on a chromosome. It is based on recombination frequency between two loci during meiosis: 1 cM corresponds to a 1% chance that a crossover occurs between the loci in a single meiosis, i.e., a recombination frequency of 0.01. Consequently, a distance measured in centimorgans reflects genetic linkage rather than an exact physical length in base pairs.

Because recombination rates vary across the genome and among species, the physical distance represented by 1 cM is not constant. In humans, a rough average is about 1 to 2 million base pairs per cM, but this varies widely and can be much shorter or longer in different regions, especially in recombination hotspots and near centromeres.

A unit of genetic distance called the Morgan is related: 1 Morgan equals 100 cM, and the human genome has a total genetic length of roughly 3,000–3,500 cM.

Centimorgans are used in genetic linkage mapping, genome-wide association studies, and genetic genealogy to estimate how closely loci are inherited together and to locate disease genes or map ancestry. Because recombination frequency cannot exceed 50%, loci on separate chromosomes are effectively 50 cM apart (unlinked). Distances in cM are additive on a given linkage map, but the conversion to physical distance (bp) varies with region and species.