Linkage is the tendency of two or more than two genes to stay together during the inheritance. It exists in two forms i.e. cis and trans.

The linkage group refers to the group of genes physically linked together. The number of linkage group represents the haploid number of chromosomes in a species, i.e. the number of linkage group can be correlated as the number of homologous pairs.

The linkage maps are constructed by using recombination frequencies between genes which is determined by the test cross. The distance is measured from centi-morgan (cM), it represents the unit of the map.

Recombination frequency is the measure of genetic linkage; it should not exceed 50%. The maximum limit of recombination is 50%.

Linkage can be broken by the process of homologous recombination (crossing over) which occurs at the pachytene stage of prophase I.