We have contructed a two–dimensional map of 243 markers on the X chromosome. The average distance between markers ordered by two recombinants is 5.4 centiMorgans(cM), which is reduced to 3.2 cM using a less stringent criterion of one recombinant. Map resolution is enhanced by replacing the usual reference marker format with a 2D format, and the two–recombinant rule is more conservative than the lod 3.0 criterion for order. Taken together, crossover mapping and the 2D format produces maps with greater reliability and higher resolution than maps constructed using currently accepted standards. This first high–density crossover–based map of an entire human chromosome provides a model for integrating physical and genetic maps.
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