The mapping of genes underlying either simple mendelian or complex traits can be broken down into a number of distinct stages—initial detection of the locus in a genome scan, determination of the most likely map location for the gene, and finally fine mapping of the locus. A number of experimental strategies for mapping genes in experimental organisms are available and their relative merits have been reviewed recently (1). Whatever strategy is chosen, an essential prerequisite for any gene identification project is the ability to construct a high resolution genetic map around the locus of interest.