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How To Make A Baby With Three Parents
How To Make A Baby With Three Parents
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Phụ đề (264)
0:00
On April 6th, 2016, a baby boy was born in Mexico.
0:03
That’s not surprising; babies are born every day all over the world.
0:07
But this kid was special.
0:08
A lot of us have two parents to thank for our existence.
0:11
But this boy was co-authored by a whole team of scientists,
0:14
and had three genetic parents.
0:16
And he’s not the only one! Despite the ethical debates
0:19
and complex lab techniques involved,
0:21
more three-parent babies have been born since then.
0:24
There are some excellent reasons
0:25
why you’d need a bonus genetic parent.
0:27
But as I’ll explain, there’s still a lot we
0:30
need to learn about the science underlying it—
0:32
especially as those babies grow up.
0:34
[♪INTRO]
0:37
Let’s start with the basics.
0:39
To make a two-parent baby, you need sperm and an egg.
0:42
These two cells collide, fuse,
0:44
and combine their DNA to make a full genome.
0:46
Most of that DNA is nuclear DNA,
0:49
which is contained in a cell’s nucleus.
0:50
It codes for things like eye color, hair color, and height—
0:54
plus lots of invisible processes that keep your body running.
0:57
But that isn’t the only kind of DNA you have.
0:59
You’ve probably heard of the mitochondria
1:01
as the powerhouse of the cell.
1:02
Cue flashbacks to high school biology.
1:04
But powerhouses would be more accurate,
1:07
since each of your cells contains
1:09
hundreds to thousands of mitochondria.
1:11
They help break down glucose and create ATP,
1:13
which are energy packets that your cells use for everything
1:16
from muscle contractions to chemical reactions.
1:19
But mitochondria also have their own unique DNA,
1:22
called you guessed it mitochondrial DNA.
1:25
Scientists aren’t positive how mitochondria
1:27
ended up with this genetic contraband.
1:29
But one of the most popular hypotheses is
1:31
that some time over a billion years ago,
1:33
a bacterial cell and a more complex
1:36
host cell loved each other very much,
1:38
so the bacterial cell moved into the host cell,
1:41
helping it metabolize better, and eventually became a part of it.
1:44
Fast forward to today, and that love story left you
1:47
with 37 extra genes, all related to cellular energy.
1:50
So going back to that two-parent baby,
1:53
both the sperm and the egg come to the party
1:55
with nuclear DNA and mitochondrial DNA.
1:57
But, soon after fertilization, the hundred or so
2:00
paternal mitochondria are destroyed, meaning that only the
2:04
mitochondria from the egg cell get passed to the baby.
2:06
And it’s probably a good thing the paternal
2:08
mitochondria don’t make the party’s guest list,
2:11
because sperm are super bad at proofreading their mitochondrial DNA,
2:15
leading to boatloads of mutations.
2:17
Mutations in DNA aren’t inherently bad, but having more of them
2:21
generally means that you have a higher chance of harmful mutations.
2:24
So all the mitochondria— and mitochondrial DNA—
2:27
a baby gets came from the egg, making it a gift from mom to you.
2:31
But if that egg happens to have
2:33
mitochondria with harmful DNA mutations,
2:36
those mutations can lead to mitochondrial diseases.
2:39
That’s not something any parent wants to give their child.
2:41
To make matters more complicated,
2:43
a parent might have a mix of healthy and
2:45
mutated mitochondrial DNA throughout their cells,
2:48
which is called heteroplasmy.
2:50
And even if the parent has so few mutated mitochondria that they
2:54
don’t have disease symptoms, their eggs can tell a different story.
2:57
Some of their eggs might have a ton of the mutated
3:00
mitochondrial DNA, while others might have almost
3:03
exclusively healthy mitochondria. It’s really luck of the draw.
3:06
Then there’s the people who only have mutated mitochondria,
3:09
which is called homoplasmy.
3:11
That pretty much guarantees that their eggs will
3:13
have mutated mitochondrial DNA and their
3:16
offspring will develop a related mitochondrial disease.
3:19
Unless, of course, you get a fertility lab
3:21
involved in your baby making. And a third parent.
3:24
Using mitochondrial replacement techniques, or MRT,
3:27
scientists can keep the original two parents’ nuclear DNA
3:31
but swap in a donor’s mitochondrial DNA.
3:33
That means all the obvious stuff we usually associate with
3:36
genetic inheritance, like hair or eye color, is coming from
3:40
the two parents who contributed nuclear DNA.
3:42
And the donor mitochondrial DNA contributes to critical
3:45
behind-the-scenes things like cellular energy production.
3:48
There are different kinds of MRT being explored,
3:51
but two techniques are most popular. And just for simplicity,
3:54
I’ll use “mom,” “dad,” and “donor” to refer to the
3:57
three sources of DNA involved in these procedures—
4:00
though fertility clinics help all kinds of families.
4:03
One method is the pronuclear transfer.
4:05
For this, two eggs are fertilized by the dad’s sperm.
4:08
One egg is the mom’s and the other is the donor’s.
4:11
After fertilization, the nuclear material gets scooped out of both eggs,
4:14
and the stuff from the mom’s egg is put into the donor’s.
4:17
That creates an embryo that has the mom’s
4:19
nuclear DNA but the donor’s mitochondrial DNA.
4:22
If everything looks a-okay, then the embryo can
4:25
be transferred into a uterus to get growing!
4:26
But that whole destroying a nucleus thing does mean
4:30
one fertilized egg is sort of destroyed in the process,
4:33
and that can be a bit uncomfortable for some people.
4:36
Another method is the maternal spindle transfer.
4:38
Here, the eggs’ nuclear material are swapped before fertilization.
4:42
That creates an egg with the mom’s spindle apparatus,
4:44
which are the bits that move chromosomes around,
4:47
and the donor’s mitochondria, which is then
4:49
fertilized with the dad’s sperm.
4:50
Then, the same rules apply. If it passes inspection,
4:53
it can be delivered to a uterus for growing
4:55
This was the method used for the three-parent baby in 2016,
4:58
since it avoids the ethical concerns around
5:01
discarding already fertilized human eggs.
5:03
So that covers the how of someone getting a bonus
5:06
genetic parent. An egg-stra parent, if you will.
5:09
And while mitochondrial replacement techniques open the door for
5:12
certain groups of three to have children genetically
5:14
related to all of them, the main use so far has been to help two-parent
5:18
couples avoid passing down severe mitochondrial diseases.
5:21
That was the case for that MRT baby born in Mexico in 2016.
5:25
His chromosomal mother was a heteroplasmic
5:27
carrier for something called Leigh syndrome.
5:29
Leigh syndrome, also called Leigh’s disease,
5:31
is a heritable mitochondrial disease that causes
5:34
nerve cell death and is usually fatal to children by 3 years old.
5:37
Unfortunately, this mother had already lost two children to
5:40
Leigh’s disease, in addition to having four miscarriages.
5:43
And when the researchers sampled mitochondrial DNA from her eggs,
5:46
nearly 100% carried the mutation that causes Leigh’s syndrome.
5:50
So MRT was pretty much her only option to avoid passing down
5:54
the condition while maintaining a genetic relationship to her child.
5:57
And the spindle transfer procedure
5:59
led to the birth of a healthy baby boy.
6:01
He was the first MRT baby, but he certainly wasn’t the last.
6:04
At the time we filmed this video, 8 MRT babies had been born in
6:08
the UK after their mothers were confirmed to have pathogenic
6:11
mitochondrial DNA and went through these fertility treatments.
6:14
But mitochondrial diseases aren’t the only reason
6:16
scientists are working with families to create MRT babies.
6:19
They’re also researching more general causes of infertility.
6:22
But before we get to that, here’s a quick ad break.
6:27
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7:11
Some researchers think that in cases where the cause of infertility
7:14
isn’t clear, the problem could be hiding in the cytoplasm of the egg.
7:18
Since MRT plops one egg’s nucleus into a donor egg, that fresh egg
7:22
and cytoplasm might have what’s needed to support pregnancy.
7:25
To test that hypothesis, a 2023 paper published
7:28
in the journal of Fertility and Sterility investigated whether
7:32
MRT using the spindle transfer method could benefit
7:35
25 couples who struggled with infertility in Greece.
7:38
Over the course of the study, 19 three-parent embryos
7:41
were transferred. In the end, 6 babies were born.
7:44
/Maybe/ that means MRT can help with mystery infertility.
7:48
But considering this was a small pilot study and 19 hopeful
7:51
couples were still left childless, it’s definitely not the only answer.
7:55
Plus, that was one of several studies to reveal a major flaw in MRT:
7:59
the chromosomal mother’s mitochondria can be stubborn as heck.
8:02
Because these embryologists are working with tiny single egg cells,
8:06
some mutated mitochondria can hitchhike into the donor’s egg
8:09
when the mom’s nucleus is transferred, and then
8:12
out-replicate the donor’s healthy mitochondria over time.
8:15
One of those 6 babies born to couples in Greece had less than
8:18
1% maternal mitochondrial DNA when it was a wee blastocyst,
8:22
but that spiked up to around 30 to 50% at birth
8:25
based on samples of cells from different tissues.
8:28
Scientists call that bounceback reversion.
8:30
And while it wasn’t a huge concern for this infertility study,
8:33
since the parents all had healthy mitochondrial DNA,
8:36
it’s a major concern when MRT is being
8:38
used to prevent mitochondrial diseases.
8:40
Reversion even happened with that first MRT baby born in Mexico.
8:44
At birth, some of his samples contained over
8:47
9% of his mom’s mutated mitochondrial DNA.
8:50
But, luckily, it seems like 9% isn’t enough
8:53
for someone to get symptoms of Leigh syndrome.
8:55
The mix of two parents’ mitochondrial DNA in one cell has
8:58
actually been a concern since experiments in the 1990s.
9:01
Back then, scientists were considering whether a technique
9:03
called cytoplasmic transfer might help with infertility.
9:06
In cytoplasmic transfer, the cytoplasm
9:09
and all the stuff inside—including mitochondria—
9:11
from a healthy donor egg is added to the intended parent’s egg.
9:15
The idea was that some fresh cytoplasm might
9:17
cancel out whatever was going wrong fertility-wise.
9:20
If we’re being nitpicky, these were
9:20
technically the first three-parent babies.
9:23
But it seems like having a mix of two types of mitochondrial DNA
9:26
might be dangerous, even if there aren’t pathogenic mutations.
9:30
We don’t have exact records of how many babies were
9:33
born from this research, but we think it’s about 30 or so.
9:36
And we know that at least two or three of those children ended up
9:39
with serious complications with their chromosomes or development.
9:43
Because of those cases, the scientific community now
9:45
seems to agree that cytoplasmic transfer isn’t safe in humans.
9:48
Plus, studies involving mice also suggest that even mixing
9:52
two healthy mitochondrial DNA sources can cause
9:55
cognitive impairment and behavioral abnormalities.
9:58
Regulatory bodies across the world have struggled with how to
10:01
balance the potential benefits of MRT against these kinds of safety
10:04
concerns from tinkering with reproductive genetics in the lab.
10:07
In 2015, the United Kingdom became the first country to explicitly
10:11
legalize MRT while addressing the challenge of scientific oversight.
10:15
They have a regulatory authority that grants
10:17
clinics licenses to perform the procedure and
10:20
reviews each potential patient on a case-by-case basis.
10:23
And only one fertility clinic currently can use MRT.
10:26
Essentially, the country decided that highly
10:28
regulating the procedure was the best way forward.
10:31
In stark contrast is the United States.
10:33
After those cytoplasmic transfer safety concerns,
10:36
the FDA decided in 2001 that any future egg
10:39
experimentation would require their approval.
10:41
And Congress also practically banned the clinical
10:44
trials necessary for that FDA approval.
10:46
So while MRT isn’t technically banned in the US,
10:50
there isn’t a clear path forward either.
10:52
The majority of countries around the world are
10:54
taking a “wait and see” approach, like Singapore.
10:57
Or they haven’t really considered regulating MRT
10:59
beyond some murky laws, like Mexico and Ukraine.
11:02
Because of this lack of standardization, it’s hard to say
11:05
exactly how many three-parent babies have been born in total.
11:08
We mentioned those 8 MRT births in the UK,
11:10
and the fertility study in Greece that led to 6 babies.
11:13
At least 7 babies have been born using mitochondrial
11:16
replacement techniques in Ukraine, and let’s not
11:18
forget that first MRT baby who was born in Mexico.
11:21
That’s around 22 babies. And if you want to include the 30 or so
11:25
that were conceived from cytoplasmic transfer back in the 90s, then
11:29
you’ve maybe got 50 humans walking around with three people’s DNA.
11:33
But again, nobody’s really tallying—aside from us, just now,
11:37
with incomplete information! So it’s hard to know for sure.
11:40
The science of making a three-parent baby is still in its infancy.
11:44
And even with the challenges of keeping track of these babies and
11:47
the risk of reversion, scientists are hopeful about the future of MRT.
11:51
At the time the paper came out, in July of 2025, those 8 babies
11:55
born in the UK were between 5 months and 2 years old.
11:58
None of them had any major or permanent health concerns—
12:01
even though one of them had around
12:03
16% mutated mitochondrial DNA at birth.
12:06
The first ever MRT baby, who was born in Mexico,
12:09
is only nine years old now, so it will be a while before scientists
12:12
can accurately assess the long-term safety of these techniques.
12:15
But since the future of MRT could involve addressing infertility,
12:18
helping non-traditional families grow, or avoiding fatal
12:21
mitochondrial diseases, it’s probably worth the wait.
12:25
[♪OUTRO]
