Final answer:
Harmful alleles persist due to various reasons, such as the low probability of carriers mating for recessive alleles, heterozygote advantage in certain environmental conditions, and the delayed onset of symptoms for some dominant lethal alleles like Huntington's disease.
Step-by-step explanation:
Harmful alleles persist in populations for several reasons. Despite most mutations being deleterious, harmful alleles can survive through various mechanisms. For instance, a recessive allele may continue to exist in a population because it is only expressed when two copies are present, and if it is rare, the chances of two carriers mating are low. However, if carriers do mate, there is a 25% chance their offspring will express the harmful trait. This rarity does not provide a strong selection pressure against it, allowing the allele to remain in the gene pool at low frequencies.
In some cases, such as with the sickle-cell allele, individuals who are heterozygous (carrying one copy of the mutation and one normal allele) have a survival advantage in malaria-prone regions, which offsets the negative effects of being homozygous for the mutation. This balanced selection helps maintain the allele in the population. Similarly, certain dominant lethal alleles can persist if they do not impact an individual until after they have reproduced, as with Huntington's disease, which often does not manifest until middle age, after affected individuals have had children.