Lethal allele

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 can be recessive, dominant, conditional, perinatal, or postnatal after an extended period of apparently normal development depending on the gene or genes involved.

Lethal alleles may specifically refer to embryonically lethal alleles, in which the fetus will never survive to term. Such alleles are a cause of non-Mendelian patterns of inheritance, such as the observation of traits in a 2:1 ratio.