You Can Inherit Mitochondrial DNA from Both Parents! | SciShow News


♪ Everybody knows that the DNA you inherit from
each biological parent is an even split… I mean, hopefully everybody knows: you get
half from the egg and half from the sperm that combines to make you. Well, almost. 37 genes are found in your mitochondria, those
little energy factories of cells. And that mitochondrial DNA only comes from
the egg. At least, that’s what we thought. Earlier this week, an international team of
researchers announced that they’d found strong evidence that mitochondrial DNA can
be inherited from both parents. Which could change what we know about not
only disease inheritance, but like… human history as a whole. It all started with a 4-year-old boy that
doctors suspected of having a mitochondrial disorder —a disease caused by mutations in
mitochondrial DNA. They’re tricker to diagnose and treat than
other genetic diseases because every cell in the body has several hundred mitochondria, and not all of them have the same DNA. Because mitochondrial disorders are complicated
and don’t always show up in tissue samples or fluid tests, the only way to diagnose one
with absolute certainty is with DNA sequencing. And when they sequenced this boy’s mitochondrial
DNA, they found a peculiar pattern of mutant genes. His mother and sisters had the same pattern,
which is exactly what you’d expect, and since none of them have a mitochondrial disorder,
the mutations don’t seem to explain his condition. But the pattern just stood out so much that
the team decided to test the boy’s grandparents, too. That was when they made a discovery that was
previously thought to be impossible: only some of his mother’s mutation pattern matched
her mother’s. The rest clearly matched up to the boy’s
grandfather. Even stranger, when they tested the grandfather’s
female relatives, they discovered that his pattern of mitochondrial mutations could also
be traced to both of his parents. To confirm their findings, the researchers
recruited two other, unrelated families with suspected mitochondrial disorders, and had
their mitochondrial DNA sequenced. And astonishingly, they found similar patterns. Some family members had inherited mitochondrial
variants from just their mothers, and some had inherited them from both parents. This isn’t the first study to suggest bi-parental
inheritance of mitochondria. Back in 2002, Danish researchers published
a report of a man with a mitochondrial disorder he appeared to have inherited from his father. Their results were independently verified
by Harvard researchers, who also uncovered potential evidence that mitochondrial DNA
could recombine, or mix just like regular DNA. But in the past decade and a half, no other
cases have been reported despite quite a bit of effort to find them. So the whole idea of bi-parental mitochondrial
inheritance — and especially questions of recombination — has remained a contentious
topic in genetics circles. This time, though, the researchers made absolutely
certain that their results were correct: they looked at dozens of people in three multi-generation
families, used incredibly sensitive testing methods, and had new DNA samples re-sequenced
at two other labs, just to be sure. And they wouldn’t have found any of this
if they’d stopped looking once they sequenced the boy’s mom and sisters, which is what many doctors would have done
since the mutations were inherited as expected and don’t appear to be the cause of his
ailment. If the idea that people can sometimes inherit
mitochondria from both parents doesn’t seem like a big deal, consider that mitochondrial DNA is one of
the best ways we have to trace the history of our species over centuries. Since it doesn’t change much from generation
to generation, the small changes that do show up in isolated groups —like populations of people living on different
continents— can be used to track where people came from
and how they moved around. And the changes are thought to happen at a
somewhat reliable rate, allowing us to estimate how long ago groups of people split from others. But all the math involved assumes that mitochondrial
DNA comes only from the egg—which, it seems, isn’t always the case. In fact, based on the findings, the authors
of the paper suspect babies might get some mitochondria from sperm as often as one in
every 5,000 births. And if that’s true — or if it turns out
that mitochondrial DNA can recombine, as those Harvard researchers suggested — then all
bets are off. The study authors, for their part, don’t
think their findings completely change things— there isn’t enough evidence, yet, to overturn
decades of research into human origins. But it’s early in the discovery process,
and it could be years before we really understand how all this messes with ancestry analyses
or our understanding of human history. More immediately, this study will likely affect
how doctors approach the genetic component to treating mitochondrial disorders, especially
since the team has a hypothesis for how this bizarre inheritance happened. At first, both parents contribute mitochondrial
DNA to a developing fetus, but when the egg is fertilized, a mitochondrial-killing mechanism
makes sure any mitochondria from the sperm get wiped from the slate. The researchers think that there might be
a mutation in a nuclear gene — that is, a gene somewhere in the main
set of chromosomes we think of as a person’s genome — that’s stopping that key step in development. If such a mutation exists, it could be passed
down through a family, leading to the mitochondrial DNA pattern they saw. The researchers are hoping further studies
will reveal exactly how the mitochondrial DNA from sperm survives. Knowing that will help us understand how commonly
it happens, and it may help us figure out how to stop it from happening— perhaps preventing
some mitochondrial disorders. It could even reveal a way to prevent disease-causing
mitochondria from the egg from being passed on. But all of that is speculation at this point. Until those future studies are done, we won’t
know whether the new findings will change our understanding of the history of humanity. But either way, they’re definitely teaching
us something new about our cells. Thanks for watching this episode of SciShow
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