Lethal Genes - Genes which affect the viability of an organism are called as lethal genes and the phenomenon is called as lethality. If the lethal effect is dominant and immediate in expression, all individuals carrying the gene will die and the gene will be lost.

Some dominant lethals, however, have a delayed effect so that the organism lives for a long time, e.g. Huntington's disease. Recessive lethals carried in the heterozygous condition nave no effect but may come to expression when matings between carriers occur. The phenotypic ratio is 2: 1.