LINKAGE

It is the occurrence of two or more genetic loci in such close physical proximity on a chromosome, that they are more likely to assort (or segregate) together than independently during meiosis. Linkage occurs because crossing over does not usually take place between loci that are close to each other. The closeness of genetic linkage is expressed in centiMorgans (abbreviated cM). For example loci that are separated by crossing over in l% of gametes are 1 cM apart.

Linkage is used for more precise determination of the genotype, the determination of the pattern of inheritance and in gene mapping. It is also useful in prenatal diagnosis, carrier detection and in presymptomatic diagnosis of certain diseases.