Lethal Alleles
A lethal allele's phenotype, when expressed, causes the death of an organism. Lethal alleles arise when a mutation to a normal allele disrupts the function of an essential gene. Without this essential gene, the organism dies. Lethal alleles can be embryonic or postnatal.
Embryonic lethals cause the death of the fetus, and fertility studies are often required in order to positively determine that an embryonic lethal exists. An example of an embryonic lethal is the AY allele in mice (seen in the diagram on the right). This allele causes a yellow phenotype and is dominant to the a+ allele, which causes the grayish agouti color seen in many wild mice. However, embryos that are homozygous for the AY allele die before they are born. Thus, the live progeny from an AYa+ x AYa+ mating show a 2 yellow: 1 agouti phenotypic ratio.
Postnatal lethal alleles cause abnormalities in the progeny that cause them to die early on in development. An example is parrot jaw, which is observed in cattle. The phenotype results in a deformed mouth so the calf is unable to eat correctly and eventually starves to death.
Lethal alleles can be dominant or recessive and can be sex linked or autosomal. If the allele is dominant, then both homozygous dominant and heterozygous individuals will die. If it is a recessive allele, then only the homozygous recessive individuals will die.