On April 6th, 2016, a baby boy was born in Mexico.

That’s not surprising; babies are  born every day all over the world. But this kid was special.

A lot of us have two parents  to thank for our existence. But this boy was co-authored  by a whole team of scientists, and had three genetic parents. And he’s not the only one!  Despite the ethical debates and complex lab techniques involved, more three-parent babies  have been born since then.

There are some excellent reasons why you’d need a bonus genetic parent. But as I’ll explain, there’s still a lot we need to learn about the science underlying it— especially as those babies grow up. [♪INTRO] Let’s start with the basics. To make a two-parent baby,  you need sperm and an egg.

These two cells collide, fuse, and combine their DNA to make a full genome. Most of that DNA is nuclear DNA, which is contained in a cell’s nucleus. It codes for things like eye  color, hair color, and height— plus lots of invisible processes  that keep your body running.

But that isn’t the only kind of DNA you have. You’ve probably heard of the mitochondria as the powerhouse of the cell. Cue flashbacks to high school biology.

But powerhouses would be more accurate, since each of your cells contains hundreds to thousands of mitochondria. They help break down glucose and create ATP, which are energy packets that  your cells use for everything from muscle contractions to chemical reactions. But mitochondria also have their own unique DNA, called you guessed it mitochondrial DNA.

Scientists aren’t positive how mitochondria ended up with this genetic contraband. But one of the most popular hypotheses is that some time over a billion years ago, a bacterial cell and a more complex host cell loved each other very much, so the bacterial cell moved into the host cell, helping it metabolize better, and  eventually became a part of it. Fast forward to today, and  that love story left you with 37 extra genes, all  related to cellular energy.

So going back to that two-parent baby, both the sperm and the egg come to the party with nuclear DNA and mitochondrial DNA. But, soon after fertilization, the hundred or so paternal mitochondria are  destroyed, meaning that only the mitochondria from the egg  cell get passed to the baby. And it’s probably a good thing the paternal mitochondria don’t make the party’s guest list, because sperm are super bad at  proofreading their mitochondrial DNA, leading to boatloads of mutations.

Mutations in DNA aren’t inherently  bad, but having more of them generally means that you have a  higher chance of harmful mutations. So all the mitochondria— and mitochondrial DNA— a baby gets came from the egg,  making it a gift from mom to you. But if that egg happens to have mitochondria with harmful DNA mutations, those mutations can lead  to mitochondrial diseases.

That’s not something any parent  wants to give their child. To make matters more complicated, a parent might have a mix of healthy and mutated mitochondrial DNA throughout their cells, which is called heteroplasmy. And even if the parent has so few  mutated mitochondria that they don’t have disease symptoms, their  eggs can tell a different story.

Some of their eggs might have a ton of the mutated mitochondrial DNA, while others might have almost exclusively healthy mitochondria.  It’s really luck of the draw. Then there’s the people who  only have mutated mitochondria, which is called homoplasmy. That pretty much guarantees that their eggs will have mutated mitochondrial DNA and their offspring will develop a  related mitochondrial disease.

Unless, of course, you get a fertility lab involved in your baby making. And a third parent. Using mitochondrial  replacement techniques, or MRT, scientists can keep the original  two parents’ nuclear DNA but swap in a donor’s mitochondrial DNA.

That means all the obvious  stuff we usually associate with genetic inheritance, like hair  or eye color, is coming from the two parents who contributed nuclear DNA. And the donor mitochondrial  DNA contributes to critical behind-the-scenes things like  cellular energy production. There are different kinds of MRT being explored, but two techniques are most  popular.

And just for simplicity, I’ll use “mom,” “dad,” and “donor” to refer to the three sources of DNA involved in these procedures— though fertility clinics  help all kinds of families. One method is the pronuclear transfer. For this, two eggs are  fertilized by the dad’s sperm.

One egg is the mom’s and the other is the donor’s. After fertilization, the nuclear  material gets scooped out of both eggs, and the stuff from the mom’s  egg is put into the donor’s. That creates an embryo that has the mom’s nuclear DNA but the donor’s mitochondrial DNA.

If everything looks a-okay, then the embryo can be transferred into a uterus to get growing! But that whole destroying  a nucleus thing does mean one fertilized egg is sort  of destroyed in the process, and that can be a bit  uncomfortable for some people. Another method is the maternal spindle transfer.

Here, the eggs’ nuclear material  are swapped before fertilization. That creates an egg with  the mom’s spindle apparatus, which are the bits that move chromosomes around, and the donor’s mitochondria, which is then fertilized with the dad’s sperm. Then, the same rules apply.  If it passes inspection, it can be delivered to a uterus for growing This was the method used for  the three-parent baby in 2016, since it avoids the ethical concerns around discarding already fertilized human eggs.

So that covers the how of someone getting a bonus genetic parent. An egg-stra parent, if you will. And while mitochondrial replacement  techniques open the door for certain groups of three to  have children genetically related to all of them, the main use  so far has been to help two-parent couples avoid passing down  severe mitochondrial diseases.

That was the case for that MRT  baby born in Mexico in 2016. His chromosomal mother was a heteroplasmic carrier for something called Leigh syndrome. Leigh syndrome, also called Leigh’s disease, is a heritable mitochondrial disease that causes nerve cell death and is usually  fatal to children by 3 years old.

Unfortunately, this mother had  already lost two children to Leigh’s disease, in addition  to having four miscarriages. And when the researchers sampled  mitochondrial DNA from her eggs, nearly 100% carried the mutation  that causes Leigh’s syndrome. So MRT was pretty much her only  option to avoid passing down the condition while maintaining a  genetic relationship to her child.

And the spindle transfer procedure led to the birth of a healthy baby boy. He was the first MRT baby, but  he certainly wasn’t the last. At the time we filmed this video,  8 MRT babies had been born in the UK after their mothers were  confirmed to have pathogenic mitochondrial DNA and went through  these fertility treatments.

But mitochondrial diseases aren’t the only reason scientists are working with  families to create MRT babies. They’re also researching more  general causes of infertility. But before we get to that,  here’s a quick ad break.

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Some researchers think that in  cases where the cause of infertility isn’t clear, the problem could be  hiding in the cytoplasm of the egg. Since MRT plops one egg’s nucleus  into a donor egg, that fresh egg and cytoplasm might have what’s  needed to support pregnancy. To test that hypothesis, a 2023 paper published in the journal of Fertility and  Sterility investigated whether MRT using the spindle  transfer method could benefit 25 couples who struggled  with infertility in Greece.

Over the course of the study,  19 three-parent embryos were transferred. In the end, 6 babies were born. /Maybe/ that means MRT can  help with mystery infertility. But considering this was a  small pilot study and 19 hopeful couples were still left childless,  it’s definitely not the only answer.

Plus, that was one of several studies  to reveal a major flaw in

MRT: the chromosomal mother’s  mitochondria can be stubborn as heck. Because these embryologists are  working with tiny single egg cells, some mutated mitochondria can  hitchhike into the donor’s egg when the mom’s nucleus is transferred, and then out-replicate the donor’s  healthy mitochondria over time. One of those 6 babies born to  couples in Greece had less than 1% maternal mitochondrial DNA  when it was a wee blastocyst, but that spiked up to around 30 to 50% at birth based on samples of cells from different tissues. Scientists call that bounceback reversion.

And while it wasn’t a huge concern  for this infertility study, since the parents all had  healthy mitochondrial DNA, it’s a major concern when MRT is being used to prevent mitochondrial diseases. Reversion even happened with that  first MRT baby born in Mexico. At birth, some of his samples contained over 9% of his mom’s mutated mitochondrial DNA.

But, luckily, it seems like 9% isn’t enough for someone to get symptoms of Leigh syndrome. The mix of two parents’  mitochondrial DNA in one cell has actually been a concern since  experiments in the 1990s. Back then, scientists were  considering whether a technique called cytoplasmic transfer  might help with infertility.

In cytoplasmic transfer, the cytoplasm and all the stuff inside—including mitochondria— from a healthy donor egg is added  to the intended parent’s egg. The idea was that some fresh cytoplasm might cancel out whatever was  going wrong fertility-wise. If we’re being nitpicky, these were technically the first three-parent babies.

But it seems like having a mix  of two types of mitochondrial DNA might be dangerous, even if there  aren’t pathogenic mutations. We don’t have exact records  of how many babies were born from this research, but  we think it’s about 30 or so. And we know that at least two or  three of those children ended up with serious complications with  their chromosomes or development.

Because of those cases, the  scientific community now seems to agree that cytoplasmic  transfer isn’t safe in humans. Plus, studies involving mice  also suggest that even mixing two healthy mitochondrial DNA sources can cause cognitive impairment and behavioral abnormalities. Regulatory bodies across the  world have struggled with how to balance the potential benefits of  MRT against these kinds of safety concerns from tinkering with  reproductive genetics in the lab.

In 2015, the United Kingdom became  the first country to explicitly legalize MRT while addressing the  challenge of scientific oversight. They have a regulatory authority that grants clinics licenses to perform the procedure and reviews each potential patient  on a case-by-case basis. And only one fertility  clinic currently can use MRT.

Essentially, the country decided that highly regulating the procedure was the best way forward. In stark contrast is the United States. After those cytoplasmic transfer safety concerns, the FDA decided in 2001 that any future egg experimentation would require their approval.

And Congress also practically banned the clinical trials necessary for that FDA approval. So while MRT isn’t technically banned in the US, there isn’t a clear path forward either. The majority of countries around the world are taking a “wait and see” approach, like Singapore.

Or they haven’t really considered regulating MRT beyond some murky laws, like Mexico and Ukraine. Because of this lack of  standardization, it’s hard to say exactly how many three-parent  babies have been born in total. We mentioned those 8 MRT births in the UK, and the fertility study in  Greece that led to 6 babies.

At least 7 babies have been  born using mitochondrial replacement techniques in Ukraine, and let’s not forget that first MRT baby who was born in Mexico. That’s around 22 babies. And if  you want to include the 30 or so that were conceived from cytoplasmic  transfer back in the 90s, then you’ve maybe got 50 humans walking  around with three people’s DNA.

But again, nobody’s really  tallying—aside from us, just now, with incomplete information!  So it’s hard to know for sure. The science of making a three-parent  baby is still in its infancy. And even with the challenges of  keeping track of these babies and the risk of reversion, scientists  are hopeful about the future of MRT.

At the time the paper came out,  in July of 2025, those 8 babies born in the UK were between  5 months and 2 years old. None of them had any major  or permanent health concerns— even though one of them had around 16% mutated mitochondrial DNA at birth. The first ever MRT baby, who was born in Mexico, is only nine years old now, so it  will be a while before scientists can accurately assess the long-term  safety of these techniques.

But since the future of MRT could  involve addressing infertility, helping non-traditional  families grow, or avoiding fatal mitochondrial diseases, it’s  probably worth the wait. [♪OUTRO]