Question

Lethal mutations were first detected by
A. Morgan
B. Muller
C. L.Cuenot
D. Bateson
E. None of the above

Answer 1

A type of mutation in which the effect(s) can lead to death or dramatically decrease the expected lifespan of a mutation-carrying organism. Brachydactyly, for example, is lethal when the genetic mutation is demonstrated in homozygous recessive individuals during infancy.

Complete Answer:
- Lethal alleles are alleles that cause the death of the organism that carries them (also referred to as lethal genes or lethals). Typically, they are a product of gene mutations that are necessary for growth or development.
- Depending upon the gene or genes involved, lethal alleles can be recessive, dominant, or conditional. Lethal alleles, although they typically manifest early in life, may cause death of an organism prenatally or any time after birth.
- Lucien Cuénot first discovered lethal alleles in 1905 when researching the inheritance of coat colour in mice. For the determination of coat colour, the agouti gene in mice is largely responsible. In each hair of the rodent, the wild-type allele produces a combination of yellow and black pigmentation. In colour, this yellow and black combination can be referred to as 'agouti'.
- Mice with a much brighter, yellowish colour result in one of the mutant alleles of the agouti gene. A 1:1 ratio of yellow and dark grey offspring was obtained when these yellow mice were crossed with homozygous wild-type mice. This suggested that the yellow mutation was dominant, and for the mutant allele, all the parental yellow mice were heterozygotes.
- Geneticists assumed that mutations would only change the appearance of a living organism until Mendel 's Laws were rediscovered. It was discovered, however, that a mutant allele could cause death. It may result in a lethal phenotype when an important gene is mutated.
- The homozygote and heterozygote for the allele will display the lethal phenotype if the mutation is triggered by a dominant lethal allele. If a recessive lethal allele causes the mutation, the homozygous for the allele may have the lethal phenotype. Most lethal genes are recessive.
- Recessive lethal alleles, unless a certain protein production threshold is preserved, do not cause death in the heterozygous form. The protein production does not reach the threshold in the homozygous form, causing death.

The correct Answer is option (C) L.Cuenot.

Note: Cystic fibrosis, Tay-Sachs disease, sickle-cell anaemia, and brachydactyly are examples of diseases caused by recessive lethal alleles. The disease of Huntington is caused by a dominant lethal allele and, while it is not defined as lethal, it is typically lethal in that for several years before death, the person suffers progressive neural degeneration and mental decline.