Life-Span Extension

Scientists have discovered that the genetic constitution of an organism can have a significant effect on its longevity. In humans, only a small fraction of the life span seems to be under genetic control, but this estimate could be wrong for many reasons. Geneticists who study humans are interested to know how much of the variation in individual longevity is controlled by the genes of the individual and how much is environmental. To answer this question, geneticists estimate something called heritability. For example, if on average, two Americans chosen at random will differ in life span by ten years, then heritability is a way of saying how much of that variation is due to environment and how much is due to genetic influences. (In this case, environment means the external environment as well as the stochastic differences within the animal, tissue or cell). The best current estimates are that genetics has a significant, but modest, effect on overall longevity, explaining only about 20 percent of the variation.

This estimate does not mean that aging is controlled by 20 percent of the genes, or that 20 percent of life is genetically programmed and the rest determined by environment. There are good reasons to suppose that centenarians have been blessed with very good genes (actually we all have the same genes, but centenarians have good versions called alleles). So in a centenarian, forty or fifty years of life could result from genetic effects. It's impossible to be certain, but many groups are looking for these genes.