OK.

So we are going to start in 1891,

when a German scientist was looking through a microscope

at insect cells.

And he saw something kind of funny.

At the center of the cells, there was this dark stuff.

No one had ever seen it before.

And he noticed that as the cells would multiply and divide,

it would go into some of the new cells

but not the others.

He didn't know what it was, so he gave it a really great name.

He called it the "X element."

(Laughter)

And he was like, "We'll just fill in that X later."

And then, fast-forward 10 years later,

and there is an American scientist,

and she is looking through her microscope,

also at insect cells.

And she sees something funny.

There's more of this dark stuff.

And it's kind of tiny,

it's hanging out near the X element.

And eventually, someone was like,

"Well, if that one thing's called X,

should we call this other thing Y?"

And like that, (Snaps fingers)

the sex chromosomes had been discovered.

So chromosomes,

you probably all know what they are,

but I will tell you anyways.

They're made up of DNA --

everything has it, it's the blueprint of life,

we've got rats, we've got trees,

we've got insects, we've got humans.

And in the case of human chromosomes,

geneticist Melissa Wilson broke it down for me like this.

(Audio) Melissa Wilson: Typically, you'll get one copy of every chromosome

from your genetic mom

and one copy of every chromosome from your genetic dad,

and we have 22 of these

that you get one copy from mom and one copy from dad.

And then there's a 23rd pair, X and Y.

Molly Webster: So while all the other chromosomes are numbered,

one through 22,

we do not call X and Y 23.

I like to think that they are waiting for, like, a LeBron James to come along.

But in this instance, they were like,

"We're just going to keep the letters,

and then we'll give them a title."

They called them the sex chromosomes.

Now I would wager that in the United States,

these are the most well-known chromosomes

for one simple fact:

that we say X equals "girl," and Y equals "boy" --

that they are responsible for sex.

And -- and I had to learn this --

but when I'm talking about "sex" here,

I'm talking about the way biology gives us gonads,

which are our ovaries and our testes --

I'm not talking about gender, which is how we identify.

And so, as a reporter at the show --

"Radiolab," the audio documentary program I work for --

I was like, what's up with these sex chromosomes?

You know, that's kind of my job, I think things are weird,

and then I get to call people about them

and ask questions, and then hopefully they answer.

And in this case, a lot of people answered.

And in the two years I had of reporting on X and Y,

as part of "Gonads," the series on sex and gender

I ended up doing for "Radiolab,"

I found out that these two chromosomes live in a world that is unexpected,

a little unsettling;

where things that I thought were facts

were, like, twisted in ways I hadn't seen before.

And the world goes so far beyond the boundaries of sex,

I was like,

"Maybe we should all talk about this."

So, you're you all,

we're all going to talk about it.

And for me,

the true story of X and Y starts with their name.

So within years of being discovered,

these two little chromosomes had acquired more than 10 different names.

There was diplosome and heterochromosome

and idiochromosome,

and most of the names had to do with their structure, their shape,

their size.

And then there was "sex chromosome,"

which they had been given because of the fact

that we had started seeing that the X would go with the females,

and the Y would often go with the males.

But scientists were like,

"Do we really want to call them sex chromosomes?"

And science historian Sarah Richardson is the one who told me this story.

(Audio) Sarah Richardson: For three decades, scientists were like,

"You should not call them the sex chromosomes.

The X and Y have many functions,

and you wouldn't assume that a single chromosome

controls a single trait.

Imagine calling one chromosome the 'urogenital chromosome,'

or the 'liver chromosome.'"

MW: Scientists, if you dig into the history --

it's really cool, you should --

were hesitant to, like, commit to such a specific name

and such a powerfully connotated name.

There was a fear that it would actually be really limiting --

maybe to science, maybe to society --

but the fear was in the room.

And you can see they ended up getting "sex chromosome" --

it's like a pretty juicy title,

it popularized genetics, you know?

But in the 100-year history since we settled on that name,

you can see it starts to get a little complicated.

So around 1960 --

this is going to be our first stop

on the complicated world of the sex chromosomes --

so around 1960,

we had discovered that you could be XYY.

They discovered an XYY man.

And to digress a little here,

it turns out that the model of "X equals girl and Y equals boy"

is really simplistic.

You can actually be a whole bunch of different combinations of X and Y,

giving you, like, different types of biological sex.

You could be two Xs and two Ys together.

You could be four Xs, you could be five Xs,

you could be XO.

And so I thought that was pretty crazy,

because I was like,

"Wow, this really upends a model of biological sex

I think most of us in this room have been taught."

So a few years after they realized that you can be XYY,

researchers go to a prison in Scotland,

and they do genetic analysis of a bunch of the male prisoners.

And they find a number of people who are XYY.

And according to Sarah:

(Audio) SR: They just rushed to publish a theory

suggesting that this extra Y chromosome

could explain criminality in some men.

MW: Yeah.

So the logic goes like this:

By this point, we're thinking Y is male.

We think male is aggressive,

so Y must be aggression.

If you've got an extra Y, you must be crazy.

And like, we went nuts with this theory.

We called it the supermale,

they started scanning more prisoners,

serial killers, boys.

And in all seriousness,

there was actually a suggestion that we consider aborting XYY fetuses.

So in 1980,

this theory pretty much toppled, for a number of reasons.

One,

there had been this really large study

that basically showed there was no connection

between Y and violence,

I think we all saw that coming.

And then, there was one other thing.

(Audio) SR: Going back and looking at those original findings

in that high-security psychiatric institution,

they had also found a high number of individuals

with an extra X chromosome.

So these are XXY, as opposed to XYY.

(Audio) MW: Really?

(Audio) SR: Yeah. Now, they never claimed

that the individuals with an extra X chromosome

were superfemales.

They never investigated whether they had higher rates of violence.

MW: Seems like kind of an oversight.

I don't know.

But I think it's interesting,

because what you see is if you start looking at these chromosomes

through the lens of sex,

what naturally falls in place behind