Gene linkage is a phenomenon that occurs when two or more genes situated at different loci on the same chromosome segregate together during anaphase of meiosis and become distributed into the same gamete. The loci are usually close to each other. Linked genes segregate together unless recombination occurs. Gene linkage is an exception to Mendel's second law of inheritance, the law of independent assortment. The distance between loci can be estimated by calculating the recombination frequency(%) of offspring in a genetic cross and is measured in centiMorgan(cM) or map unit(m.u.).