Genetic conditions that lead to mortality are common in older adults than younger people Humans go through several stages throughout their lives, including growth, development, reproduction, and senescence. Senescence is defined as the deteriorative biological changes that organisms experience as they age, ultimately leading to their death. These changes include a low metabolism, a weak immune system, memory loss, vision problems and hearing loss. Senescence begins in humans during the post-reproductive years. However, gerontology research has shown that late-reproducing individuals have a longer lifespan than early-reproducing individuals. However, this does not indicate that senescence is inevitable. All organisms experience senescence, but at different rates and times. Many genetic diseases such as Alzheimer's disease and Huntington's disease are prevalent in older individuals, and symptoms appear in middle adulthood. The causes of genetic diseases and disabilities in elderly individuals are explained by three evolutionary theories: antagonistic pleiotropy theory, mutation accumulation theory, and disposable soma theory. These theories suggest that favorable natural selection and the heavy allocation of resources to somatic maintenance during the reproductive period decrease the chances of genetic diseases in younger individuals. The theory of antagonistic pleiotropy was introduced by Charles Williams in 1957. Pleiotropy occurs when one gene influences multiple traits in the body. The theory of antagonistic pleiotropy states that natural selection favors genes that promote fitness in early life despite their detrimental effects in later life (Gavrilov and Gavrilova 341). Deleterious genes that occur in young adults are not passed on to... middle of paper... nescence is the result of a combination of antagonistic pleiotropy theory, mutation accumulation theory, and soma theory uses and throw. The three evolutionary theories state that natural selection is weak and ineffective in supporting reproduction, growth, survival, and somatic repair during the post-reproductive years causing genetic diseases in older than younger individuals. When biological processes break down, older individuals develop a weak immune system that can no longer fight off deleterious genes and mutations that lead to death. Furthermore, evolutionary theories have paved the way for new research in gerontology that has led to the development of new genetic theories of aging. The definitive cause of genetic diseases in older individuals has yet to be determined, but all theories suggest that senescence is an inevitable process that all organisms experience.