Often traits in plants and animals are influenced by many genes rather than a single locus [Falconer and MacKayFalconer and MacKay1996, for an excellent general review]. These traits are termed quantitative traits and the loci that control these traits quantitative trait loci, abbreviated henceforth as QTLs. An important goal in genetics and breeding is to identify and characterize QTLs, especially those that contribute to variation in quantitative traits both within and between populations or species. The recent advances in molecular biology have allowed the construction of genetic linkage maps based on molecular markers. Such genetic linkage maps can span the genome at regular intervals. The experimenter can then look for correlations between these mapped markers and the trait of interest in controlled breeding experiments to gain insight into the regions of the genome that control the trait.