A technique, used in constructing a physical map, for selecting contiguous overlapping clones from a DNA library and thus reconstructing the order of genes along a segment of chromosome. For example, one can effectively ‘walk’ in either direction from a known marker gene to identify adjacent genes, a technique that is useful in fine-structure mapping of a genome. Essentially, the initial clone containing the marker gene is fragmented and each fragment subcloned for use as a DNA probe to identify other clones containing adjacent and overlapping segments. In turn, these adjacent segments are fragmented and subcloned and used to probe for further overlaps, and so on. The cloned segments can then be placed in order corresponding to that on the chromosome. In a refinement of the technique, called chromosome jumping, only the ends of segments are identified, allowing the investigator to ‘jump’ over the middle regions. This speeds up the process, and also is a way of bypassing stretches of repetitive DNA, which are not amenable to chromosome walking.