Question

4. In traits that exhibit complete dominance, why is it that most genetic disorders are recessive? That is, how does natural selection allow them to persist in the population when the traits they confer are extremely deleterious?

Answer 1

Most common genetic disorders are recessive in nature because recessive alleles can persist in population in the hetrozygote organisms without showing any deleterious effects and without any reduction in survival or reproduction in the organism or in other words dominant allele masks the effect of trait produced by recessive allele. So these organisms transfer the recessive allele to the next generation and natural selection allows these alleles to persist in the population even though these alleles are extremely deleterious in homozygous form. On the other hand dominant disorders are less common as organisms carrying allele for disease eliminated by natural selection as these alleles are not masked by effect of another allele as in case of recessive disorders.