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Hank: The basis behind human reproduction is pretty simple -- an egg cell from your mom and a sperm cell from your dad each provide half of their genetic material to complete the 23 pairs of chromosomes of DNA that make up your genome.

This is the complete set of genetic instructions that makes you unique. But there’s a whole other set of genetic instructions inside your cells, called mitochondrial DNA. And in some cases, a third person can donate that mitochondrial DNA as a part of fertility treatment.

Our genome encodes for more than 20,000 individual genes. Together, they’re responsible for things like your looks, how you respond to disease, and powering each individual cell within your bodies. That kind of DNA is stored in the form of chromosomes inside the nuclei of your cells.

Inside your cells, you also have structures called mitochondria, also known as the powerhouse of the cell because they’re mainly responsible for converting energy from food into cellular energy. But your mitochondria actually have a whole genome of their own -- 37 genes that come only from your mother. See, right after fertilization, the mitochondrial DNA from the sperm cell gets destroyed. So mitochondrial DNA is only inherited from mom. But OK, why do you have mitochondrial DNA in the first place?

Well, according to what’s known as endosymbiotic theory, mitochondria originally started out as bacteria with circular DNA. Eventually, the bacteria were eaten up by more complex cells, which formed what we know as eukaryotes. This new bacterial DNA turned out to be useful for the eukaryotic cell, helping the cell manage its energy in exchange for a nice environment to thrive.

Over the years, mitochondrial DNA hasn’t changed much, so scientists can use it to trace maternal genetics back through evolutionary history. They’ve used mitochondrial DNA to trace domesticated dogs to their wolf ancestors, and even found evidence of interbreeding between Neanderthals and humans.

But because mitochondrial DNA hasn’t seen much variation over the years, certain genetic mutations can cause rare, but devastating, diseases. LHON, for example, is a disease that causes vision loss. Then there’s Pearson Syndrome, which causes pancreas issues and anemia, and can lead to diabetes. And Leigh Syndrome is a neurological disorder that leads to loss of motor and mental function. Mitochondrial DNA mutations have also been linked to more common diseases, like diabetes, Alzheimer’s, and Parkinson’s.

If a female has disease-causing mutations in her mitochondrial DNA, there isn’t much she can do about it, but there is a way for her to make sure that those mutations don’t get passed on to her children: mitochondrial DNA donation, where her mitochondrial DNA can be swapped out for healthy DNA from a donor. It also works as a fertility treatment, if she has mutations that cause fertility issues. The very first method for doing this, called cytoplasmic transfer, was discovered back in 1997.

It involves removing the cytoplasm, all the components of a cell except for the nucleus, from a healthy donor egg, then inserting it into the mother’s egg before in vitro fertilization. In 2015, two more techniques for mitochondrial DNA transfer were developed. One is called spindle transfer, which is kind of like the opposite of cytoplasmic transfer: the nucleus from the mother’s egg is inserted into a healthy donor egg, which is then fertilized by sperm in vitro.

Then there’s pronuclear transfer, where the nucleus from an already fertilized egg is transferred into a healthy donor egg, which gets transplanted into the mother’s womb. When there’s a mitochondrial DNA donor involved, you can identify all three parents with a genetic test -- which is why some people refer to babies conceived this way as “three-parent babies. ” Which isn’t that big a deal, on its own.

But as often happens when it comes to new treatments that involve human embryos, there’s a lot we don’t know. There might be future complications that come from being born with genes from three parents. There are a few people around today whose mitochondrial DNA was donated -- but only a few. Not enough to do comprehensive studies.

We can test this in other animals, though, and one study in mice found that when donated mitochondrial DNA wasn’t a close match for the mother’s original mitochondrial DNA, it led to accelerated aging of cells, metabolic disorders, and obesity. The babies seemed healthy when they were young, but they developed more issues as they grew older.

And since mitochondrial DNA doesn’t change much with each generation, any complications from a genetic mismatch would eventually be passed on to the child’s future children, too. Still, they were mice -- mice that were very inbred, which could have affected the results of the experiment. We don’t know if something similar would happen in humans. But if it does, then it might be important to have a donor that’s a good match -- like with a blood transfusion or organ transplant.

So we still have a lot to learn about the potential risks of mitochondrial DNA donation. But it’s also a promising way for mothers to make sure they don’t pass on mutations that can cause a lot of harm -- and in some cases, the only way they might be able to have biological children at all. And all this because a few billion years ago, a cell happened to munch on some bacteria. And they lived inside of us forever!

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