Definition of Lethal Mutation
The mutation is any change that has occurred in the DNA which may include deletion, insertion of bases, etc. The lethal mutation is that kind of mutation that produces a lethal allele that is responsible for the death of the cell or even the individual. When a gene is mutated and is expressed, it leads to the premature death of an organism that is carrying it.
The homozygotes are killed only in the case of recessive lethal alleles whereas both the homozygotes as well as the heterozygotes are killed when the alleles are dominant. Death does not happen immediately but may take months or years to happen.