Humans with one copy of the mutant CF gene have a slightly diminished capacity to lose salt and water through their membranes and are thus relatively protected from the most devastating complications of cholera (this can be demonstrated using genetically engineered mice). Here too a mutation in a gene can have a dual and circumstantial effect—potentially beneficial in one copy, and lethal in two copies. Humans with one copy of the mutant CF gene may thus have survived cholera epidemics in Europe.
