map unit [L, mappa, napkin, unus, one] , an arbitrary unit of measure used to express the distance between genes on a chromosome. It is calculated from the percentage of recombinations that occur between specific genes so that 1% of crossing over represents one unit on a genetic map, or approximately the number of new combinations that can be detected. The measurement is accurate only for small distances because double crossovers do not appear as new recombinations. Also called map distance. See also morgan.
