LINKAGE DISEQUILIBRIUM

Linkage disequilibrium can be defined as a deviation from gametic equilibrium: at equilibrium the frequency of the AB gamete is the product of the frequencies of the component alleles. It has been described in a number of regions of the human genome in particular in the HLA complex, the beta-globin region, the apolipoprotein genes A1-C3-A4 region and around the genes for many recessive diseases. Recombination tends to reduce any initial linkage disequilibrium but genes very closely linked remain far from equilibrium for hundreds of generations. A number of mechanisms could produce linkage disequilibrium, the most important are: population admixture, random drift and founder effect, selection effect.