Any plan that shows the positions of genes, genetic markers, or other landmarks along the length of a chromosome. There are essentially two complementary types of map: linkage maps, which give the relative positions of genetic sites (loci) determined by studies of how frequently recombination occurs between the loci; and physical maps, which show the arrangement of the chromosomal material, whether it be in the form of banding patterns produced by staining (a type of cytological map), the order and distance between restriction enzyme cleavage sites (a restriction map), or the sequence of bases in the DNA. Maps of either type can be constructed in various ways, depending on such factors as the type of organism, the complexity of its genome, and the amount of pre-existing data. Accumulated data for the chromosomes of many species of organism are now held in databases and available freely via the Internet for geneticists and others worldwide.