Some studies on the genetics of human longevity have looked at small, homogeneous populations - this makes the task easier for a number of reasons. Here is another example: "The advantage of working with a homogeneous population is, you're reducing the variances that can be associated with the environment. [Mennonites and Amish] don't drink, don't smoke. Most do some sort of physical activity. They don't sit around working on a computer all day. ... 5 percent of healthy Amish octogenarians have 'haplogroup X,' a genetic pattern within the mitochondria, which are the regions of cells that generate energy and help guard against deterioration. Haplogroup X is generally found in only 2 percent of Europeans, from whom the Amish descended. In the University of Miami study, only 3 percent of the control group - Amish people who had made it to 80 but suffered from significant disease or disability - had the genetic variant. ... Mitochondria have their own DNA, which is passed down from the mother only. This unique chromosome has variations, called haplogroups. Nine such haplogroups have been well characterized in people of European descent ... But only haplogroup X was found to be prevalent among healthy aged people in the University of Miami study." As we know, some people have better mitochondria; a side-effect of technologies that allow us to replace damaged mitochondrial DNA throughout the body would be the ability to upgrade to a better haplotype. That benefit is tiny, however, compared to the benefit of removing the damage that contributes to degenerative aging.