A unit for measuring distance between genes (or other loci) on a chromosome according to the frequency of recombination between due to crossing over. A distance of 1 m.u.—or 1 centimorgan (1 cM)—corresponds to a recombinant frequency of 1%, i.e. the two genes recombine once in every 100 meioses. Map units are used in constructing linkage maps; they measure relative genetic distance between loci, not absolute physical distance. They are additive over short distances, but are not reliable for distantly linked genes because of the possibility of multiple crossovers. Compare crossover value.