Correct option is D
Recessive lethal alleles are never completely eliminated from a population because they persist in heterozygous carriers. A recessive lethal allele (aa) typically causes death when inherited in homozygous form, but heterozygous individuals (Aa) can survive and pass on the allele to the next generation.
Lethal Alleles Persist in Populations:
Heterozygote Carriers:
- Individuals with one normal allele (A) and one lethal allele (a) (i.e., Aa) survive and reproduce, maintaining the allele in the gene pool.
- Since lethal effects occur only in homozygous recessive (aa) individuals, the allele is not completely eliminated.
Natural Selection and Hidden Alleles:
- Natural selection removes homozygous recessive (aa) individuals, but since heterozygotes do not express the lethal phenotype, selection does not effectively eliminate the allele.
- The lower the allele frequency, the rarer aa individuals become, making it difficult for selection to remove the allele entirely.
Balanced Polymorphism (Heterozygote Advantage):
- In some cases, heterozygotes (Aa) have a selective advantage, maintaining the allele in the population.
- Example: Sickle cell anemia (HbS allele) – Homozygous individuals (HbS/HbS) develop a severe disease, but heterozygotes (HbA/HbS) are resistant to malaria, which preserves the allele in populations exposed to malaria.
Mutation and Genetic Drift:
- Lethal alleles can reappear due to mutations. Even if natural selection removes affected homozygotes, new mutations can reintroduce the allele.
- In small populations, genetic drift can randomly influence allele frequencies, allowing lethal alleles to persist.
Additional Information
Option (1) Lethal alleles are always conditional in nature (Incorrect):
- Some lethal alleles may be conditional (e.g., only lethal in certain environments), but this is not a general rule.
Option (2) Lethal alleles have selective advantage (Incorrect):
- Lethal alleles themselves do not have a selective advantage, but in some cases, heterozygotes carrying the allele do.
Option (3) Lethal alleles protect organisms from other deleterious mutations (Incorrect):
- While some heterozygous conditions confer protection, not all lethal alleles protect against mutations.
