Because a crossover between two loci on non-sister chromatids of homologous chromosomes can occur anywhere along the interval separating them, the probability that two genes are separated by recombination is proportional to the physical distance between them: genes far apart recombine frequently, genes close together rarely. This proportionality makes observed recombination frequency a measurable proxy for distance, formalized in the centimorgan (equivalently, the genetic map unit), defined as the distance between genes for which one product of meiosis in one hundred is recombinant — that is, a one percent recombination frequency. The concept belongs to classical and linkage genetics, and applying it pairwise across all loci on a chromosome yields a genetic linkage map giving the relative order and spacing of genes.
Because a crossover between two loci on non-sister chromatids of homologous chromosomes can occur anywhere along the interval separating them, the probability that two genes are separated by recombin…