Rare alleles and recessive traits

Most mutations are recessive as it is easier to disrupt the function of a gene than to create a new gain-of-function. They are normally also relatively rare in a population. Thus, for a recessive human disease gene, a heterozygote won’t experience any negative selection pressure. Only the homozygote form will be selected against, but because they are rare, the frequency of the double mutant will be very low. Thus, the mutant allele is maintained in a population by carriers – providing the population is large-enough.