Question

Many deleterious genes are recessive and therefore are expressed only in the homozygote. Many of them are also lethal and many of them have also been present in the human gene pool for hundreds of thousands and possibly even millions of years. How do you explain the persistence of these genes over such long periods of time in the face of such intense selective pressure against them.

Answer 1

Sometimes having just one copy of the recessive allele actually provides a competitive advantage. The classic case is a mutation that, when homozygous recessive, is lethal. But when heterozygous (one dominant and one recessive allele) confers some partial resistance to malaria.

Answer 2

@blues thank you =)