genetic map
A genetic map is like a subway map of a chromosome: it shows the order of genes and how far apart they are, drawn not from a ruler but from how often the genes get shuffled apart during meiosis. The more often two genes recombine, the farther apart they are placed on the map.
Distances on a genetic map are measured in centimorgans, where one centimorgan equals one percent recombination frequency. By comparing recombination frequencies among many pairs of genes, geneticists can deduce their linear order and add up the small distances to span a whole chromosome.
Genetic maps differ from physical maps, which measure actual distances in base pairs of DNA. The two correlate but are not identical, because recombination is more common in some chromosome regions than others. A given number of centimorgans can therefore correspond to different amounts of DNA in different places.
Because long distances are underestimated by single recombination counts, accurate maps are built by chaining together many short, reliable intervals rather than measuring one large gap directly. The first such map was made by Alfred Sturtevant for fruit-fly genes in 1913.