So everybody knows you get half your DNA from your mother and half your DNA from your father, right? Well, except for that portion of your DNA that comes only from your mother that resides in your mitochondria. Let's back up, way back, to the moment of conception. When sperm and an egg come together, the sperm donates its DNA but not much else. This means that the main volume of the cell comes from the egg. And this volume includes all of its organelles. This means that all the mitochondria in a fertilized egg come from the mother. So it's kind of a science cliche to call mitochondria the powerhouses of the cell. But that's really what they are. Mitochondria produce ATP, a cell's source of chemical energy. Mitochondria have a pretty wild history. These things look a lot like single-celled prokaryotes. They've got these membrane structures. And they've got these circular bits of DNA that code for a lot of the same things that proteobacteria code for. This thing looks a lot like its own little organism. So what is it doing in our cells? Well, the endosymbiotic theory proposes that mitochondria developed from a symbiotic relationship between two long ago organisms. One was a proteobacteria, which became our mitochondria. These proteobacteria were probably engulfed by an early cell that looked a lot like our eukaryotic cells, probably because the bigger cell was trying to eat it. But that proteobacteria survived. And because it was producing all this energy which the larger cell could harness, it allowed the largest cell to live and thrive in a greater diversity of environments. So over time, this symbiotic relationship turned from one big cell which has eaten a proteobacteria into our eukaryotic cells and our mitochondria. So all that was to tell you that mitochondria have their own DNA. And when fertilization occurs, you get all your mitochondria from the maternal egg cells. And so you get all of your mitochondrial DNA from your mother. So mitochondrial DNA can be a really powerful tool for tracking genetic lineages. Because siblings will get identical mitochondrial DNA from their mother, it's a really accurate way to test for biological siblings. Mitochondrial DNA testing is also used by the military to identify skeletons found in old war zones by accurately linking them back to living relatives. So mitochondrial DNA is also a really powerful tool to trace female lineages. And you can do this far back into history. And you watch how different populations moved. And you can trace lots of populations back to single individual females. And that's awesome, and normally what people pull out as the coolest thing, and normally what people would stop and talk to you about. But there is something that I think is possibly even cooler than that about mitochondrial DNA. And we're going to talk about that instead. So there's a paper that came out in 2012 entitled, "Mitochondria, Maternal Inheritance, and Male Aging." This paper takes the idea that because mitochondrial DNA is only passed down through females, it can only be acted upon by selection that directly acts on females. So let's say you have a mutation in your mitochondrial DNA, which makes your hair green. If a man has this mutation, he will have green hair. But he will not pass it on to his children. If a woman has this mutation, she will have green hair. And she will pass it on to her children who will also have green hair. Now, let's say that the green hair mutation also makes you infertile. If this infertility affects women, then they will never be able to pass on their mitochondrial DNA. And the trait will die out. However, if that infertility only affects men, then women with green hair will continue to pass on the trait. Now, the green-haired sons will all be infertile, but their green-haired daughters will not be. And they, too, will continue to pass on this trait. In this way, the maternally-inherited DNA has picked up a mutation that only affects men. Weird. Now, the researchers who wrote this paper looked at this idea as it relates to aging. And they used my favorite model organism, fruit flies. And they summed up their findings into four key points. Number one, mitochondrial genomes harbor mutations affecting male but not female aging. Number two, there appear to be numerous mutations in mitochondrial DNA affecting male aging. Number three, maternal inheritance of mitochondria, thus, has dramatic effects on male life history. And number four, this process should generally contribute to sex differences in longevity and aging. I think this needs some more research to be anything conclusive. But it is such a freaking cool idea. DNA passed down through one sex is specifically affecting the other sex. That is so freaking cool. And before you start yelling at me in the comments about, well, what about X-linked diseases and things on the sex chromosomes? OK. I know. I know. But come on. This is wicked awesome. Ah. So cool. Go forth. Do science. There's a paper that came out in 2012 entitled, "Mitochondria, Maternal Inheritance, and Male Aging." And that is a tongue twister. And I dare you to say that five times fast. Mitochondria, maternal inheritance, and male aging, mitochondria, maternal inheritance, and male aging, mitochondria, maternal inheritance, and male aging, mitochondria, maternal inheritance, and male aging, mitochondria, maternal inheritance, and male aging.