The sweet smell of fruit doesn’t normally send rats running.
But when researchers paired the orange-cherry-almondy scent of the chemical acetophenone with a
painful electric shock, lab rats quickly learned to fear it.
Along the way, extra neurons sprouted in their noses and in the smell-processing center of
their brains, making them super-sensitive to the scent.
This result isn’t shocking.
What is surprising is that the rats' pups – and their pups' pups – were also startled
by the smell of acetophenone and had the same extra neurons as their fathers, despite never
having been introduced to either their dads or the fruity scent before.
But how could the pups have inherited something their fathers learned?
Basic genetics tells us that only DNA gets passed along to offspring; characteristics
like memories, scars, or giant muscles, can’t get passed on since acquiring them doesn’t
alter the genetic code.
But it turns out that instilling fear in the rats did trigger genetic changes - not in
the DNA sequence itself, but instead, in how that code was read and used in the rats’
bodies.
In every cell, biological machinery constantly translates DNA into the proteins needed to
carry out vital processes.
Chemical switches attached to the DNA turn genes on and off or up and down, telling the
machinery which proteins to produce and in what quantities.
These switches, called “epigenetic tags,” are why a kidney cell looks and acts differently
than a skin or nerve cell, even though the two cells’ DNA is identical.
But the switches in any one cell aren’t set in stone: teaching those rats to fear
that fruity smell switched one of their smell-sensing gene into overdrive.
Researchers don’t know all the places in the rats’ bodies where this switch got flipped,
but they know it happened in one key set of cells: the rats’ sperm cells, which would
one day pass along this tweaked genetic material, making the next generation of rats super-sensitive
to acetophenone.
Rodents aren’t the only creatures demonstrating this weird type of inheritance.
In Överkalix, Sweden, boys who suffered through tough winter famines went onto have super-healthy
sons, with extremely low risks of heart disease and diabetes.
And their sons’ sons had the same excellent health, living an unbelievable 32 years longer,
on average, than the grandsons of boys who hadn’t gone hungry.
To be clear, this does not mean we should start starving our kids for the benefit of
future generations – scientists don’t even know yet exactly which switches the Swedish
famines flipped.
While we have been able to connect specific epigenetic changes to health effects in mice,
we’re a long way off from being able to make those connections in humans.
That may sound like a bummer, but it’s mostly because we humans don’t live in the well-controlled
environment of a laboratory.
And for that, we should be grateful.