Explanation

Recessive lethal alleles ($aa$) cause pre- or postnatal death only in homozygotes. Heterozygotes ($Aa$) are healthy carriers, so the allele is transmitted from generation to generation unless carriers mate and, by chance, produce homozygous offspring ($aa$), which are then lost. Since most individuals carry only one copy in the heterozygous state, complete elimination of such alleles is extremely slow, especially in large populations.​

Option-by-option

1. Lethal alleles are always conditional in nature

• Conditional lethals express only under certain environments, but many classic lethal alleles are unconditional and remain in populations due to the heterozygous state, not conditional expression.

2. Lethal alleles have selective advantage

• Most lethal (loss-of-function) alleles are deleterious, not advantageous, except in rare cases (sickle cell, cystic fibrosis carriers under specific environments—not applicable generally).

3. Maintained as heterozygotes – correct

• The classic explanation: Because only $aa$ die and $Aa$ survive and reproduce, the allele persists in the heterozygous state.​

4. Lethal alleles protect organisms from other deleterious mutations

• There are few known balanced lethal systems, but generally, lethal alleles don’t protect from unrelated mutations; their persistence is due to unexpressed phenotype in carriers.

Example for clarity

• For a recessive lethal allele ($a$), with Hardy-Weinberg frequencies:

  • $AA$ (normal): p2p2

  • $Aa$ (carrier): $2pq$

  • $aa$ (lethal): q2q2 (do not survive)

• Because the lethal homozygotes are purged, but carriers remain, the $a$ allele can never be entirely removed unless all carriers are purged as well.

In summary:
Recessive lethal alleles are never completely eliminated because they are maintained in the population as phenotypically normal heterozygotes who continue to pass the allele to offspring.