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In genetics, a centimorgan (abbreviated cM) or map unit (m.u.) is a unit for measuring genetic linkage. It is defined as the distance between chromosome positions (also termed loci or markers) for which the expected average number of intervening chromosomal crossovers in a single generation is 0.01. It is often used to infer distance along a ...
There are two distinctive mapping approaches used in the field of genome mapping: genetic maps (also known as linkage maps) [7] and physical maps. [3] While both maps are a collection of genetic markers and gene loci, [8] genetic maps' distances are based on the genetic linkage information, while physical maps use actual physical distances usually measured in number of base pairs.
Where d is the distance in map units, the Morgan Mapping Function states that the recombination frequency r can be expressed as =.This assumes that one crossover occurs, at most, in an interval between two loci, and that the probability of the occurrence of this crossover is proportional to the map length of the interval.
Genetic linkage is the tendency of DNA sequences that are close together on a chromosome to be inherited together during the meiosis phase of sexual reproduction.Two genetic markers that are physically near to each other are unlikely to be separated onto different chromatids during chromosomal crossover, and are therefore said to be more linked than markers that are far apart.
Figure 1: Genetic distance map by Cavalli-Sforza et al. (1994) [1] Genetic distance is a measure of the genetic divergence between species or between populations within a species, whether the distance measures time from common ancestor or degree of differentiation. [2] Populations with many similar alleles have small genetic distances. This ...
Distance matrices are used in phylogeny as non-parametric distance methods and were originally applied to phenetic data using a matrix of pairwise distances. These distances are then reconciled to produce a tree (a phylogram, with informative branch lengths).
It is expressed as a percentage, which is equivalent to the number of map units (or centiMorgans) between two genes. For example, if 100 out of 1000 individuals display the phenotype resulting from a crossover between genes a and b, then the recombination frequency is 10 percent and genes a and b are 10 map-units apart on the chromosome.
DNA segment with three genes, showing a double recombination event. If the individual recombination rates (between A and B; and between B and C) are known, then the c.o.c. between the regions AB and BC can be calculated from the rate of double recombination.