The centimorgan (cM), named after Thomas Hunt Morgan, is the unit of genetic distance on a linkage map. One centimorgan equals a 1% probability of recombination (crossing-over) between two loci per generation. It is calculated from recombination frequency: $1\text{ cM} = 1\%$ recombination frequency. Genetic maps (linkage maps) measure distances between genes based on crossover frequency, not physical distance. Physical maps use kilobases (kb) or megabases (Mb).