Deleterious recessive alleles, such as the allele that causes cystic fibrosis, can persist in a population indefinitely. Why aren’t these rare alleles eliminated?
There are several reasons why deleterious recessive alleles, like the one causing cystic fibrosis, can persist in a population indefinitely:
1. Carriers: Individuals who carry a copy of the recessive allele but do not show symptoms of the condition can pass it on to their offspring unknowingly. This helps maintain the allele in the population even though it may be rare.
2. Genetic drift: In small populations, chance events can cause certain alleles to become more prevalent, even if they are deleterious. This can lead to the persistence of rare alleles in a population.
3. Heterozygote advantage: In some cases, carrying one copy of a recessive allele can actually provide a fitness advantage. For example, individuals who are carriers of the sickle cell allele are protected against malaria. This can help maintain the allele in the population.
4. Mutation: new deleterious alleles can arise through mutations, even if they are rare. If these mutations are not strongly selected against, they can persist in the population.
Overall, a combination of factors such as carriers, genetic drift, heterozygote advantage, and mutation can allow deleterious recessive alleles to persist in a population over time.