Discovery
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. 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 the accumulation of two mutant alleles is lethal and results in stillbirths.
Read more about this topic: Lethal Alleles
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