Editing Genetic linkage (section)

{{Short description|Tendency of DNA sequences that are close together on a chromosome to be inherited together}}
{{Redirect-distinguish|Genetic map|Gene mapping}}

'''Genetic linkage''' is the tendency of [[Nucleic acid sequence|DNA sequences]] that are close together on a [[chromosome]] to be inherited together during the [[meiosis]] phase of [[sexual reproduction]]. Two [[Genetic marker|genetic markers]] that are physically near to each other are unlikely to be separated onto different [[Chromatid|chromatids]] during [[chromosomal crossover]], and are therefore said to be more ''linked'' than markers that are far apart. In other words, the nearer two [[Gene|genes]] are on a chromosome, the lower the chance of [[Genetic recombination|recombination]] between them, and the more likely they are to be inherited together. Markers on different chromosomes are perfectly ''unlinked'', although the [[penetrance]] of potentially deleterious alleles may be influenced by the presence of other alleles, and these other alleles may be located on other chromosomes than that on which a particular potentially deleterious allele is located.<ref name="Cooper">{{Cite journal |last1=Cooper |first1=DN |last2=Krawczak |first2=M |last3=Polychronakos |first3=C |last4=Tyler-Smith |first4=C |last5=Kehrer-Sawatzki |first5=H |date=October 2013 |title=Where genotype is not predictive of phenotype: towards an understanding of the molecular basis of reduced penetrance in human inherited disease |journal=[[Human genetics]] |volume=132 |issue=10 |pages=1077–130 |doi=10.1007/s00439-013-1331-2 |pmc=3778950 |pmid=23820649}}</ref>

Genetic linkage is the most prominent exception to [[Gregor Mendel]]'s [[Mendelian inheritance#Law of Independent Assortment|Law of Independent Assortment]]. The first experiment to demonstrate linkage was carried out in 1905. At the time, the reason why certain traits tend to be inherited together was unknown. Later work revealed that genes are physical structures related by physical distance.

The typical unit of genetic linkage is the [[centimorgan]] (cM). A distance of 1 cM between two markers means that the markers are separated to different chromosomes on average once per 100 meiotic product, thus once per 50 meioses.