The probability of zygotes (i.e., cells formed by the fusion of the egg cell (in human, the ovum) and the spermatozoid) or seeds to survive and develop into an adult organism is known as viability.

Mutations in gametes (i.e., germ cells, egg or spermatozoid) may give origin to a zygote carrying a deleterious allele that can result in reduced fitness, implying a lower probability for development or survival up to a reproductive age. Since most mutated genes are recessive, a heterozygous carrier will usually not manifest the related disease phenotype. However, if such recessive mutation is also present in the other inherited allele, embryo inviability, or the onset of a hereditary lethal syndrome may occur in early or late stages of life, depending on the gene involved.

Recessive mutated alleles are therefore termed semi-lethal genes, because death will occur only in homozygote individuals. Nevertheless, if the...