Lethal Alleles

An allele that results in the death of an organism is called a lethal allele. Lethal alleles always involve essential genes, which are necessary for an organism’s survival.

When a mutation is caused by a dominant lethal allele, both homozygotes and heterozygotes that inherit the allele perish.

Dominant lethal alleles are rarely identified in populations because the affected individuals are often eliminated rapidly. A notable exception to this observation is Huntington’s disease, a neurological disorder that ultimately leads to death.

However, the onset of disease doesn’t occur until the age of 40. By this time, many affected individuals have already passed their mutant alleles to their progeny.

Unlike dominant lethal alleles, recessive lethal alleles only result in the death of homozygotes.

An example of an essential gene is agouti, which is responsible for determining coat color in mice. Agouti was first studied in 1905 by Lucien Cuénot and is depicted here with capital A for agouti.

Cuénot experimented on the wild-type allele of the agouti gene, which is functional and gives rise to a gray coat color in mice. Such a mouse is called 'agouti' in color.

A mutant allele of the agouti