Lethal alleles

Alleles that cause an organism to die are called lethal alleles, where the gene involved is an essential gene. When Mendel's Laws were rediscovered, geneticists believed that mutants 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 allele, the homozygote and heterozygote for the allele will show the lethal phenotype. If the mutation is caused by a recessive lethal allele, the homozygote for the allele will have the lethal phenotype.