Genetic linkage map shows the relative positions of genetic markers on a chromosome. The positions of genetic markers are determined based on how often the loci are inherited together rather than on the specific physical distance between them. It means that the more often the two genetic markers are inherited together the farther apart they are. In contrast, the lower frequency of being inherited together the closer the smaller the physical distance is between them.
Genetic map units are measured in terms of centimorgans (cM).