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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.
One utility of this approach is that it allows one to obtain values for distances in genetic mapping units directly from recombination fractions, as map distances cannot typically be obtained from empirical experiments. [1] The simplest mapping function is the Morgan Mapping Function, eponymously devised by Thomas Hunt Morgan.
Chromosomes display a banded pattern when treated with some stains. Bands are alternating light and dark stripes that appear along the lengths of chromosomes. Unique banding patterns are used to identify chromosomes and to diagnose chromosomal aberrations, including chromosome breakage, loss, duplication, translocation or inverted segments.
A genome map is less detailed than a genome sequence and aids in navigating around the genome. [75] [76] An example of a variation map is the HapMap being developed by the International HapMap Project. The HapMap is a haplotype map of the human genome, "which will describe the common patterns of human DNA sequence variation."
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 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.
A genetic marker is a gene or DNA sequence with a known location on a chromosome that can be used to identify individuals or species. It can be described as a variation (which may arise due to mutation or alteration in the genomic loci) that can be observed.
An additional method in comparative genomics is genetic mapping. In genetic mapping, visualizing synteny is one way to see the preserved order of genes on chromosomes. It is usually used for chromosomes of related species, both of which result from a common ancestor. [58] This and other methods can shed light on evolutionary history.