Human gene mapping.

It is now possible to map the human genome completely with a set of closely linked markers. Over 500 coding genes have been cloned and localized, as have approximately 2000 anonymous DNA fragments, most of which recognize two-allele polymorphisms that are caused by single base changes which alter the recognition site for a restriction enzyme (restriction fragment length polymorphisms). Most human chromosomes have been mapped, with markers in defined order placed approximately 10 map units apart. Chromosomes X and 21 are particularly well mapped, with over 200 probes ordered on X. The strategy during the next few years will encompass moving from a linkage map to a set of overlapping cosmid or phage clones, and finally to a complete sequence of regions of chromosomes and entire chromosomes. A complete sequence of the human genome should transform our understanding of development, the control of gene expression, and the parameters of genetic disease.