Physical Maps Using Molecular Markers - The physical maps of DNA fragments, chromosome fragments or of whole individual chromosomes are based on distances between genes or distances f genes .from centromeres or from specific molecular markers measured either in terms of base pairs (megabase pairs = 106bp) or in terms of dative physical lengths.

For instance, the distance of a gene from centromere may be expressed as a fraction of the length of the whole arm taken as unity, so that the distance of a gene from centromere will 'e indicated as 0'1, if it is located at a distance of 1/10th of the whole rm.

Sometimes, physical maps of individual loci on small chromosome fragments are also prepared, where the DNA sequences on either side of the locus are shown in the form of restriction sites for a number of restriction enzymes.