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"The Phenotypic manifestations of some mutant genes results in death of the individual earlier in prenatal or postnatal (postpartum) period prior to sexual matuarity. Such mutant genes are called Lethal genes."
When Mendel's Laws were rediscovered, geneticists believed that mutations would only alter the appearance of a living organism. However, it was discovered that a mutant allele could cause death. When an essential gene is mutated, it can result in a lethal phenotype. If the mutation is caused by a dominant lethal genotype, the heterozygote for the allele will show the lethal phenotype, the homozygote dominant is impossible. If the mutation is caused by a recessive lethal genotype, the homozygote for the allele will have the lethal phenotype.
i. They are not limited to X-Chromosome.
ii. They are of more frequent occurrence in the autosome.
iii. It involves the interaction of many genes.
iv. It might interfere with the development of some vital organ such as heart or it might upset soe vital process chemical or physiological early in development.
Types of Lethality :-
Lethal genes were first discovered by Lucien Cuénot while studying the inheritance of coat colour gene in mice. He expected a phenotype ratio from a heterozygote cross of 3 yellow:1 white, but the observed ratio was 2:1. By performing test crosses, he determined that all the yellow mice were heterozygotes and that the yellow colour coat was the dominant phenotypic trait. However, no homozygous yellow mice were obtained. In 1910, William Ernest Castle and C. C. Little reaffirmed Cuénot's discovery of a lethal gene by proving that a quarter of the offspring from crosses between heterozygotes died during embryonic development, due to failure to implant in the uterine lining. The quarter that died were the homozygous yellow mice that Cuénot did not see in his tests.
An example of lethal alleles in humans is achondroplasia, a genetic condition which causes dwarfism. Affected individuals are all heterozygotes, as two mutant alleles is lethal and results in a non-viable fetus.
Another example of a lethal allele occurs in the Manx cat. Manx cats possess a heterozygous mutation resulting in a shortened or missing tail. Crosses of two heterozygous Manx cats result in 2 offspring displaying the heterozygous shortened tail phenotype, and 1 offspring of normal tail length that is homozygous for a normal allele. Homozygous offspring for the mutant allele cannot survive birth and are therefore not seen in these crosses.