1. A combination of selective effects that on average have the same impact on the composition of the gene pool as one death; two carriers at 50% risk of dying would be the lethal equivalent of one carrier at 100% risk. 2. In the population genetics of recessive traits, lethal equivalent is expressed as twice the sum of the expected number of deaths ascribable to the genetic load. 3. Expression used of the genetic load of recessive genes in heterozygous state that if in homozygous state would cause death or carry a risk of death. The expected number of deaths from all such genes is expressed in lethal equivalent.