Apply 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 a few reasons why deleterious recessive alleles, like the one that causes cystic fibrosis, may persist in a population indefinitely:
1. Heterozygote advantage: In some cases, individuals who carry one copy of the recessive allele may actually have a reproductive advantage. This is known as heterozygote advantage. For example, carriers of the allele that causes sickle cell anemia are more resistant to malaria. This can help maintain the allele in the population, even though individuals who inherit two copies of the allele may suffer from the disease.
2. Genetic drift: In small populations, random fluctuations in allele frequencies can lead to the persistence of deleterious recessive alleles. If the allele is rare to begin with, genetic drift can prevent it from being eliminated over time.
3. Mutation and gene flow: New mutations can introduce deleterious alleles into a population, while gene flow can bring these alleles in from other populations. These processes can help maintain the allele in the population even if it may be harmful.
4. Balancing selection: In some cases, the presence of a deleterious allele may be balanced by other factors, such as environmental conditions or other genetic traits. This can help maintain the allele in the population at low frequencies.
Overall, the persistence of deleterious recessive alleles in a population is a complex phenomenon influenced by a variety of factors. While natural selection tends to eliminate harmful alleles, other factors can sometimes prevent this from happening, allowing these alleles to persist in a population indefinitely.