This is a mutation that results in the demise of the organism affected. Lethal mutations tend to be recessive, resulting in the death of an organism which is homozygous recessive. An example of such a mutation can be observed in Manx cats. Characterised by their lack of tails and longer hind legs, Manx cats carry a dominant allele (ML) that severely affects spinal development. All Manx cats are heterozygotes; cats that are homozygote for the ML allele do not survive because the allele is recessive lethal.
Mutations can also occur as a result of a dominant lethal allele, for example Huntington disease. In the case of dominant lethal only one copy of the mutated allele needs to be present to result in the death of an individual.