So this is a talk about gene drives,
but I'm going to start by telling you a brief story.
20 years ago, a biologist named Anthony James
got obsessed by the idea of making mosquitos
that didn't transmit malaria.
It was a great idea, but pretty much a complete failure.
For one thing, it turned out to be really hard
to make a malaria resistant mosquito.
James managed it, finally, just a few years ago
by adding some genes that make it impossible
for the malaria gene to survive inside the mosquito.
But that just created another problem.
Now that you've got malaria-resistant mosquito,
how do you get it to replace all the malaria-carrying mosquitos?
There are a couple options,
but plan A was basically to breed up a bunch of the new genetically-engineered mosquotos,
release them into the wild,
and hope that they pass on their genes.
The problem was that you'd have to release
literally 10x the number of native mosquitos to work.
So in a village with 10,000 mosquitos,
you release an extra 100,000.
As you might guess, this was not a very popular strategy
with the villagers.
(Laughter)
Then, last January, Anthony James got an email
from a biologist named Ethan Bier.
Bier said that he and his grad student, Valentino Gantz,
had stumbled on a tool that could not only guarentee
that a particular gene trait would not be inherited,
but that it would spread incredibly quickly.
If they were right, it would basically solve the problem
that he and James had been working on for 20 years.
As a test, they engineered two mosquitos
to carry the anti-malaria gene
and also this new tool, a gene drive,
which I'll explain in a minute.
Finally, they set it up so that any mosquitos
that had inherited the anti-malaria gene
wouldn't have the usual white eyes, but would instead have red eyes.
That was pretty much just for convenience
so they could tell just at a glance which was which.
So they took their two anti-malarial, red eye mosquitos
and put them in a box with 30 ordinary white-eyed ones
and let them breed.
In two generations, those had produced 38,000 grandchildren.
That is not the surprising part.
This is the surprising part:
given that you started with just two red-eyed mosquitos
and 30 white-eyed ones,
you expect mostly white-eyed descendents.
Instead, when James opened the box,
all 38,000 mosquitos had red eyes.
When I asked Ethan Bier about this moment,
he became so excited, tht he was literally shouting into the phone.
That's because getting only red-eyed mosquitos
violates a rule that is the absolute cornerstone of biology,
Mendelian genetics.
I'll keep this quick, but Mendelian genetics says when a male and female mate,
their baby inherits half of its DNA from each parent.
So if our original mosquito was aa and our new mosquito is aB,
where B is the anti-malarial gene,
the babies should come out in four permutations:
aa, aB, aa and Ba.
Instead, with the new gene drive,
they all came out aB.
Biologically, that shouldn't even be possible.
So what happened?
The first thing that happened was the arrival
of a gene-editing tool known as CRISPR in 2012.
Many of you have probably heard about CRISPR,
so I'll just say briefly that CRISPR is a tool that allows researchers
to edit genes very precisely, easily and quickly.
It does this by harnessing a mechanism that already existed in bacteria.
Basically, there's a protein that acts like a scissors
and cuts the DNA,
and there's an RNA molecule that directs the scissors
to any point on the genome you want.
The result is basically a word processor of genes.
You can take an entire gene out, put one in,
or even edit just a single letter within a gene.
And you can do it in nearly any species.
Okay, remember how I said that gene drives
originally had two problems?
The first is that it was hard to engineer a mosquito
to be malaria resistant.
That's basically gone now, thanks to CRISPR.
But the other problem was logistical.
How do you get your trait to spread?
This is where it gets clever.
A couple years ago, a biologist at Harvard named Kevin Esvelt
wondered what would happen if you made it so that
CRISPR inserted not only your new gene,
but also the machinery that does the cutting and pasting.
In other words, what if CRISPR also copy and pasted itself.
You'd end up with a perpetual motion machine for gene editing.
And that's exactly what happened.
This CRISPR gene drive that Esvelt created
not only guarantees that a trait will get passed on,
but if its used in the germline cell,
it will automatically copy and paste your new gene
into both chromosomes of every single individual.
It's like a global search and replace,
or in science terms,
it makes a heterozygous trait homozygous.
So, what does this mean?
For one thing, it means we have a very powerful,
but also somewhat alarming new tool.
Up until now, the fact that gene drives didn't work very well
was actually kind of a relief.
Normally when we mess around with an organisms's genes,
we make that thing less evolutionarily fit.
So biologists can make all the mutant fruit flies they want
without worrying about it.
If some escape, natural selection just takes care of it.
What's remarkable and powerful and frightening about gene drives
is that that will no longer be true.
Assuming that your trait does not have a big evolutionary handicap,
like a mosquito that can't fly,
the CRISPR-based gene drive will spread the change relentlessly
until it is in every single individual in the population.