centiMorgan

Centimorgan, abbreviated cM, is a unit of genetic map distance used in the field of genetics to quantify the probability of recombination between two genetic loci during meiosis. By definition, 1 centimorgan corresponds to a 1% chance that a crossover between the loci will occur in a single meiosis, under a given map function. It is a unit of recombination frequency, not a physical distance, and therefore does not correspond directly to base pairs.

Because observed recombination fractions are subject to interference and sex-specific differences, the relationship between cM and physical distance varies by region and species. Recombination fractions (r) range from 0 to 0.5; 1 cM is approximately 0.01 in r, but the mapping from r to cM depends on the chosen mapping function. The Haldane function assumes no interference; Kosambi accounts for some interference. In practice, genetic maps express distance in centimorgans by applying these functions to observed recombination data.

In humans, as a rough rule of thumb, 1 cM corresponds to about 1 million base pairs on average, but this varies; recombination hotspots and chromosomal position produce wide variation. Maps are sex-specific; female recombination rates tend to be higher, yielding longer genetic maps than male maps for the same genome.

Applications include constructing linkage maps, locating genes, QTL mapping, and pedigree-based studies; cM distances are additive along a chromosome on a genetic map, though physical distance per cM varies.