A lethal allele is a gene variant associated with a mutation in an essential gene, which has the potential to cause the death of an individual. In general, lethal genes are recessive because these alleles do not cause death in heterozygous individuals, which have one copy of the normal allele and one copy of the allele for the lethal disease/disorder. In recessive lethal diseases, heterozygous individuals are carriers of the recessive lethal allele and can eventually pass the 'defective' allele on to offspring even though they are unaffected; whereas dominant lethal diseases are caused by dominant lethal alleles, which only need to be present in one copy to be fatal. In consequence, the frequency of recessive lethal alleles is generally higher than dominant lethal alleles because they can be masked in carrier individuals. Some examples of human diseases caused by recessive lethal alleles include, among others, Tay-Sachs disease, sickle-cell anemia, and cystic fibrosis.
