by A.M. Benis, Sc.D., M.D. We examine how interacting genetic traits can affect the interpretation of the results of case-control studies. The basis of our analysis is a model of personality reposing on three genetic traits: sanguinity (N), perfectionism (P) and aggression (A). The model is based on classical Mendelian genetics. The genes for traits N and A are entangled, in the sense that if one of the traits is absent, then the other must be present. This entanglement of "complementary genes" is the cause of infertility in certain combinations of NPA types. We examined hypothetical case-control studies with the use of Hardy-Weinberg methodology and gene frequencies to generate representative populations of varied genetic constitution. Six different hypothetical case-control studies were replicated in seven different populations. The computed results of each case-control study were highly dependent on the genetic composition of the representative population. When the condition present in the "case group" was genetically related to either trait N or A, there was nevertheless a reciprocal difference induced in the other trait due to their entanglement, leading to possible confusion in interpretation of the results. The text is written so that it may be appropriate for the general reader having an interest in human genetics. A Glossary and a Synopsis of the NPA model are provided in the Appendix. Paperback, 80 pp., 25 figures and tables.