Genetic markers that are sufficiently polymorphic (as measured by their heterozygosities) can be used in linkage and association analyses to detect Mendelian segregation underlying disease phenotypes. Each type of analysis can either be based on a specific genetic model or not make any assumptions about the mode of inheritance of the disease. Principles underlying these methods are reviewed, and the assumptions underlying them stressed. Association analyses are more powerful, provided there is linkage disequilibrium between the marker and disease loci; however, only linkage analyses have power in the absence of such disequilibrium. For this reason, models that allow for both kinds of tests are preferred, and such models must adequately approximate the complexity of the disease being studied.