Genes are the bits of DNA that determine an organism's physical characteristics and drive a whole range of physiological processes. Longevity assurance genes are variants (called alleles) of certain genes that allow an organism to live longer. If a yeast or worm or human inherits the helpful variant of a longevity assurance gene, it has the potential to live longer (and perhaps healthier) than other members of its species that inherit other versions of that gene.