Gene linkage is a phenomenon that occurs when two or more genes situated at different loci on the same chromosome segregate together (co-segregation) 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 (RF), in %, in offspring in a genetic cross and is measured in centimorgan (cM) or map unit (m.u.). The greater the distance between loci, the greater the chance of genes being separated into different gametes due to recombination and crossing over. RF cannot be greater than 50%. Linked genes have RF of less than 50% or distance of less than 50 cM whereas unlinked genes have RF of 50% and can be more than 50 cM apart.