Lethal alleles (also referred to as lethal or lethals) are alleles that cause the death of the organism that carries them. They are usually a result of mutations in genes that are essential for growth or development.[1] Lethal alleles may be recessive, dominant, or conditional depending on the gene or genes involved.
Lethal alleles can be embryonically lethal, in which the fetus will never survive to term, or may be lethal perinatally or postnatally after an extended period of apparently normal development. Embryonically lethal alleles are a cause of non-Mendelian patterns of inheritance, such as the observation of traits in a 2:1 ratio.