that all human populations are polymorphic for the two Rh alleles. Thus, this human polymorphism must be very old, antedating the origin of modern geographical races. Yet the simple theoretical prediction is that such a poly-morphism is unstable and should have disappeared. Balance between mutation and selection The overdominant balance of selective forces is not the only situation in which a stable equilibrium of allelic frequencies may arise. Allele frequencies may also reach equilibrium in populations when the introduction of new alleles by repeated mutation is balanced by their re-moval by natural selection. This balance probably ex-plains the persistence of genetic diseases as low-level polymorphisms in human populations. New deleterious mutations are constantly arising spontaneously or as the result of the action of mutagens. These mutations may be completely recessive or partly dominant. Selection removes them from the population, but there will be an equilibrium between their appearance and removal. The general equation for this equilibrium is given in
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