Viability
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 allele mutation occurs in the dominant copy of the gene, it will suffice for disease onset or embryo inviability of the heterozygote carrier. When the allelic mutation causes death, whether at the time of fertilization or in any stage of life, it is termed lethal gene or lethal allele. An example of the action of a semi lethal gene is sickle cell anemia, a hereditary autosomal recessive blood disorder that usually causes death of homozygote carriers during late childhood or early adolescence, but is not lethal to heterozygote individuals. An instance of a lethal gene disorder is Huntington's disease (HD), an inherited autosomal dominant neuronal disorder, that may occur in early adulthood (juvenile form) or around middle age (50 years or later), affecting both heterozygote and homozygote individuals. The juvenile form of the disease is inherited from the father in the vast majority of the studied cases, whereas in the late-onset form of HD more than twice as many cases are inherited from an affected mother.
The risk rate for certain hereditary syndromes, such as Down syndrome, increases with the age of the mother at the time of conception, such rates are related to the mean maternal age (in the U.S. the mean maternal age is 29.2 years). This is known as the maternal age effect and may be related to follicular atresia (i.e., degeneration of the follicles surrounding the ovum) or genetic factors.
Achondroplasia, the most common form of human dwarfism, is an autosomal dominant trait derived in seven-eighths of the cases from a new mutation or de novo mutation in the father's gamete. Paternal age effect is associated with Achondroplasia present in the offspring of normal parents and about 80% of achondroplastic individuals die during childhood.
Viability of the offspring may be affected either by inherited mutations present in the genome of one of the parents, such as Li-Fraumeni syndrome, or breast and ovarian hereditary cancers, or by new age-related mutations occurring in the gametes of one or both parents, as exemplified above.
With regard to evolutionary mechanisms, the minimum level of viability for an organism is the ability to live long enough to successfully produce offspring that are, in turn, capable of successful reproduction.
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