Failure of two or more genes to recombine at random as a result of their location on the same chromosome pair. Among the haploid products of a cell which has gone through meiosis, two genes located in the same chromosome pair remain in their two original combinations of alleles (parental) unless an odd number of exchanges of homologous segments occurred within the interval bounded by their loci. The incidence of exchanges of homologous segments at meiosis is roughly proportional to the length of the chromosome segment between two loci. The percentage of recombinants thus provides an estimate of this length and a basis for constructing gene maps on which linked loci are arranged in linear order and spaced out in proportion to the recombination percentages between them. The analogy is with a linear map of the stations along a railway line spaced out according to the time of travel between any two or more of them.

By extension of the meaning of linkage to bacterial transduction or transformation, two bacterial genes are said to be linked when both are incorporated together in the chromosome of recipient cells more often than would be expected from mere coincidence of two separate incorporations. This is the consequence of their being frequently carried on the same transducing or transforming DNA fragment.