Is Inheritance Really All In Our Genes?

[MUSIC] We can inherit a lot from our parents. Hair
and eye color, height. But we can’t inherit everything, because some biological traits
are acquired during our lifetime. The only way to transmit biological information between
generations is in the letters of our DNA. But what if it’s not that simple? What if
our environment, and our experiences can be passed on to our children and grandchildren?
Inheritance is turning out to be much weirder than we think. [MUSIC] Every cell in your body holds an incredible
6 feet (1.8 m) of DNA. The same 6 feet of DNA, each holding identical genetic instructions.
Yet when skin cells regenerate every day, the new ones somehow “know” to become
skin cells, not bone, or muscle. Something beyond just DNA influences their destiny. This is what scientists call epigenetics,
differences in traits that aren’t due to changes in the DNA sequence. When it’s wrapped up inside the cell, tiny
chemical flags on the DNA or the proteins it’s coiled around signal the cell to turn
certain genes on or off, so they make just the right machinery to do their job. These chemical flags are rewritten every day
as organisms adapt to new environments, but scientists are seeing something strange: some
of these changes can be passed on to the next generation. Mice fed high-fat diets… get fat (unsurprisingly)
thanks to changes in the chemical flags on their DNA. But female children of these obese
mice, even though they were taken away and raised by normal-sized mothers, still ended
up 20% fatter than mice from skinny parents. In another example, male mice trained to fear
a fruity odor passed sensitivity to this smell on to their children and grandchildren, even
though their offspring had never been exposed to it. If this sounds a lot like what that guy Lamarck
was talking about, well, you’re not wrong. Before Darwin, many scientists thought acquired
traits could be passed on, but natural selection proved that wrong. But even so, scientists have since seen cases
in species from flowers to fruit flies where traits are passed on to children and grandchildren
without changing the DNA sequence. There’s just one catch. This shouldn’t
be possible. Just hours after an embryo is conceived, its
chemical flags are erased, so all the cell types in the new body can be built from a
blank slate. And cells destined to become sperm and eggs get erased a second time. At
least that’s what scientists thought. For epigenetic inheritance to work, some flags
must sneak through without being reset. This strange inheritance might even happen
in humans. During the Dutch famine at the end of WWII, children undernourished in the
womb still carried epigenetic changes more than 60 years later. And since these changes
happen in the womb, they could have a huge effect on our health as adults.
In Överkalix, Sweden, boys who lived through good harvests had sons and grandsons with
higher rates of diabetes and heart disease, while boys who lived through winter famines
had healthier grandsons – they lived an average of 32 years longer.
Strangely, girls who lived through swings of feast and famine had granddaughters with
higher rates of heart disease. That’s confusing. But human lives aren’t
easily-controlled lab studies. And that’s why some scientists doubt this
new kind of inheritance. Epigenetic changes can definitely happen between
one or two generations, but for a trait to have an effect on evolution, it has to endure
for dozens of generations. When a baby’s developing, the cells that
will make a grandchild are already present, and can be exposed to to the same environment
as the grandmother. That’s not inheritance as much as super-duper-early exposure.
For epigenetic changes to be truly inherited, they have to be rewritten in every generation,
we’d have to see them in great-grandchildren and beyond, and that’s just not clear yet. Even so, the vast majority of traits that
make us who we are are written in our DNA and it’s tough to totally rule out genetic
changes or other factors even in the cases we’ve seen. That’s the problem with studying
complex animals whose lives are the product of thousands of genes in trillions of cells.
There’s a lot going on here. But since many of our diseases are linked
to stress, diet, or environment, it wouldn’t be totally surprising to find out our bodies
are affected in ways we didn’t know about. Epigenetics is a young science, and it’s
reminding us we have a lot to learn about what makes us who we are. Stay curious. Hey guys, so epignetics, pretty cool. And
pretty weird. We just touched the tip of the iceberg of this really interesting new field,
and if you want to dig in deeper, there’s a ton of great links down in the description
for you to check out. You know what else is awesome? Physics! Crash Course Physics just
launched last week, it’s hosted by Dr. Shini Somara, and like all of the Crash Courses,
you’re gonna love it. So head on over and dig in. Stay curious!

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