Genetic diseases in humans are usually rare and recessive. Why are the frequencies of alleles that cause rare, recessive diseases (or other recessive traits, for that matter) generally much higher than the frequency of the diseases (or traits) themselves?

Recessive genetic diseases require the particular gene's two recessive alleles to be together for the disease phenotype to be expressed. The frequency of two recessive alleles coming together is low. However the recessive allele can also be present with a dominant allele. Organisms carrying such genotype are called carriers. They carry the recessive allele but do not have the disease because the other dominant allele masks the effect of the recessive allele. Frequency of recessive alleles are higher than frequency of recessive traits because the alleles are present in heterozygous condition too where the trait is not expressed.

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