Briefly, chromosome walking entails the progressive isolation and characterization of overlapping sets of genomic clones. The walk is continued in this manner until the region spanning the intervening gap has been bridged by an overlapping set of clones. While chromosome walking is technically straight forward, in practice the procedure is extremely labor intensive and ill-suited for large projects where more than a few steps are required. Recently, interest has focused on strategies for constructing physical maps of entire genomes. By definition, a physical map consists of a linearly ordered set of DNA fragments encompassing the genome or region of interest.
Physical maps are of two types, macro-restriction maps and ordered clone maps.
As the name implies, an ordered clone map consists of an overlapping collection of cloned DNA fragments. Major advantages of ordered clone maps are that they are of high resolution and directly provide the clones for further study. The immediate benefits of having a physical map are twofold. First, the physical map provides ready access to any region of the genome which can be genetically identified. Given a mutation of known genetic map location, the physical map can be used to easily isolate an overlapping collection of clones encompassing the locus of interest.