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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.
Linkage disequilibrium in asexual populations can be defined in a similar way in terms of population allele frequencies. Furthermore, it is also possible to define linkage disequilibrium among three or more alleles, however these higher-order associations are not commonly used in practice. [1]
Genes that typically stay together during recombination are said to be linked. One gene in a linked pair can sometimes be used as a marker to deduce the presence of the other gene. This is typically used to detect the presence of a disease-causing gene. [7] The recombination frequency between two loci observed is the crossing-over value.
These probabilities of recombination can be used to construct a linkage map, or a graphical representation of the location of genes and gene in respect to one another. If linkage is complete, there should be no recombination events that separate the two alleles, and therefore only parental combinations of alleles should be observed in offspring.
The linked frequency of crossing over between two gene loci is the crossing-over value. For fixed set of genetic and environmental conditions, recombination in a particular region of a linkage structure tends to be constant and the same is then true for the crossing-over value which is used in the production of genetic maps.
[citation needed] Once pairing has occurred, the crosslink can be removed and correct information introduced into the damaged chromosome by homologous recombination. Cleavage of the bond between a deoxyribose sugar in DNA's sugar-phosphate backbone and its associated nucleobase leaves an abasic site in double stranded DNA.
Within a family, linkage occurs when two genetic markers (points on a chromosome) remain linked on a chromosome rather than being broken apart by recombination events during meiosis, shown as red lines. In a population, contiguous stretches of founder chromosomes from the initial generation are sequentially reduced in size by recombination events.
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.