We examine how interacting genetic traits can affect the interpretation of case-control studies. Our model of personality reposes on three genetic traits: sanguinity (N), perfectionism (P) and aggression (A). The "complementary genes" for traits N and A are entangled: if one of the traits is absent, then the other must be present. This entanglement is the cause of infertility in certain combinations of NPA types. We examined hypothetical case-control studies with the use of Hardy-Weinberg methodology. Six different case-control studies were replicated in seven different representative 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 a reciprocal difference induced in the other trait due to their entanglement, leading to possible confusion in interpretation of the results. Synopsis, Glossary, 25 ill., 80 pp.