Given that there is a huge amount of variation at the molecular level which has no obvious functional relevance and that there must therefore be many neutral variants that will achieve significant frequencies simply by chance, a more or less arbitrary lower threshold of 1% has been proposed as the definition of polymorphic variation6. This value is mostly well above that attained by a deleterious mutation maintained in the population by mutation-selection balance. Even for completely recessive deleterious mutations, the corresponding maximum expected incidence is probably only just over 3%.
