Genetic linkage was first discovered by the British geneticists William Bateson and Reginald Punnett shortly after Mendel's laws were rediscovered.
Genetic linkage occurs when particular genetic loci or alleles for genes are inherited jointly. Genetic loci on the same chromosome are physically connected and tend to stay together during meiosis, and are thus genetically linked. This is called autosomal linkage. Alleles for genes on different chromosomes are usually not linked, due to independent assortment of chromosomes during meiosis.
Because there is some crossing over of DNA when the chromosomes segregate, alleles on the same chromosome can be separated and go to different daughter cells. There is a greater probability of this happening if the alleles are far apart on the chromosome, as it is more likely that a cross-over will occur between them.
The measure of linkage is the frequency at which two genes become seperated during recombination, when they are further apart it is more likely that they become seperated.