Define lethal allele. Explain with a suitable example the molecular basis of lethality

A lethal allele is a type of gene mutation that causes death to an organism when present in a certain genotype. Usually, lethal alleles are recessive, meaning that they cause death only when an individual inherits two copies of the lethal allele (homozygous condition). However, some lethal alleles can also act in a dominant form and cause death even when only one copy is present.

Lethal alleles generally affect essential biological processes like metabolism, development and organ formation. These alleles usually arise due to mutations that severely disrupt the function of an essential gene.

Molecular Basis of Lethality

One of the best-known classical examples of a lethal allele is found in mice involving the yellow coat color gene.

This trait is controlled by a single gene with two alleles:
  • A⁺ (normal wild-type allele)
  • Aʸ (mutant yellow allele)
The Aʸ allele causes yellow coat color and is dominant for this trait. But if a mouse has two copies of Aʸ (AʸAʸ), it dies early during development because this combination is lethal.

When two mice with one Aʸ allele and one normal allele (AʸA⁺) mate, the possible offspring genotypes are:
  • AʸAʸ (homozygous) — these die early and do not survive
  • AʸA⁺ (heterozygous) — yellow coat, healthy
  • A⁺A⁺ (homozygous) — normal coat (called agouti), healthy
Normally, when two heterozygous parents mate, the expected genotypic ratio is 1:2:1 (one AʸAʸ : two AʸA⁺ : one A⁺A⁺). But since AʸAʸ individuals die, they are not counted in the living offspring.

So, the surviving offspring ratio becomes 2 yellow (AʸA⁺) : 1 agouti (A⁺A⁺).

This is why the phenotypic ratio is 2:1, not 3:1.

At the molecular level, the lethality is due to the mutation of a gene that plays a vital role in embryonic development. In this case, the Aʸ allele is linked to the Raly gene, which controls transcription of essential proteins. The yellow coat gene (Aʸ) is a result of an insertion mutation that disrupts the normal gene structure. When two Aʸ alleles are present, the gene product becomes non-functional and embryonic development fails, causing death.

Thus, the molecular basis of lethality lies in the disruption or loss of essential gene function, leading to developmental arrest or failure of vital physiological processes.

This example clearly explains both the genetic behavior and the molecular mechanism behind a lethal allele.






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