  The logical interrelatedness of cause and effect—that is, whether a condition is necessary and/or sufficient for a given event to occur—is the approach taken to defining what makes a condition “genetic” in individuals. A strong sense of “genetic disease” is recognized when the genetic factor is both necessary and sufficient for the disease to arise “regardless of environment” (Wulff 1984), or when the genetic factor is sufficient for the disease to present “in all known environments” (Kitcher 1996)—this latter definition recognizes that, in some cases, a disease may have nongenetic as well as genetic origins (since the genetic factor is sufficient but not necessary). “Genetic susceptibility” is defined as an increased probability of disease in all known (strong sense) or some (weak sense) environments (Kitcher 1996). Note that ceteris paribus clauses referring to an assumed background of necessary, though not sufficient, genetic and environment factors are required by these definitions. Just as striking a match causes it to ignite only if it is dry and in the presence of oxygen, as we saw in the previous section, genes don’t do anything alone. This is the first of three ways in which genetic explanations are context-dependent.