  The third, and final, way in which genetic explanations are context-dependent is that they are a function of the present state of knowledge. Huntington's disease is deemed a genetic condition on both the individual and population accounts: a single mutant gene is necessary, and arguably sufficient given necessary (and standard) background conditions, for symptoms to appear in a given person; the presence and absence of disease symptoms in members of the population is accounted for in terms of the presence and absence of the mutation. This is nevertheless an epistemically relative claim. Once the relevant gene is mapped and sequenced, the mechanisms by which genetic and nongenetic factors interact to produce symptoms of the disease remain to be understood. Such causal knowledge is often obtained through the experimental manipulation of conditions beyond “normal” limits, and what conditions are exploited as possible causes in the laboratory and what conditions are kept constant as necessary background, along with pragmatic decisions about how research efforts should be expended more generally, are influenced by clinical and social, as well as scientific, contexts (Gannett  1999).[33]