Mapping

Genetic maps are a measure of distance based on recombination, which is a biological process. A different way of measuring the distance between two loci is to measure the actual number of base pairs between the loci. This is known as the physical distance, and, when many such distances are put together, it makes a physical map.

Genetic maps and physical maps are similar in that the loci will be in the same order. There is also a general correspondence of distance, in that bigger genetic distances usually correspond to bigger physical distances. The overall rule of thumb is that one centimorgan of genetic distance is about one million base pairs of physical distance. However, this comparison can vary dramatically across certain parts of chromosomes. In some areas, one centimorgan might be only 50,000 base pairs (e.g., at the ends of chromosomes, where recombination seems to be increased). In other chromosomal areas (e.g., near the centromere), one centimorgan might be five million base pairs.