The Biology of Gender, from DNA to the Brain: Karissa Sanbonmatsu (Transcript)

Learn how life experiences shape the way genes are expressed — and what that means for our understanding of gender.

Karissa Sanbonmatsu – TED Talk TRANSCRIPT

So what does it mean to be a woman? We all have XX chromosomes, right? Actually, that’s not true.

Some women are mosaics. They have a mix of chromosome types with X, with XY or with XXX. If it’s not just about our chromosomes, then what is being a woman about? Being feminine? Getting married? Having kids?

You don’t have to look far to find fantastic exceptions to these rules, but we all share something that makes us women. Maybe that something is in our brains. You might have heard theories from last century about how men are better at math than women because they have bigger brains. These theories have been debunked.

The average man has a brain about three times smaller than the average elephant, but that doesn’t mean the average man is three times dumber than an elephant or does it?

There’s a new wave of female neuroscientists that are finding important differences between female and male brains in neuron connectivity, in brain structure, in brain activity. They’re finding that the brain is like a patchwork mosaic — a mixture.

Women have mostly female patches and a few male patches. With all this new data, what does it mean to be a woman? This is something that I’ve been thinking about almost my entire life.

When people learn that I’m a woman who happens to be transgender, they always ask, “How do you know you’re a woman?”

As a scientist, I’m searching for a biological basis of gender. I want to understand what makes me me. New discoveries at the front edge of science are shedding light on the biomarkers that define gender.

My colleagues and I in genetics, neuroscience, physiology and psychology, we’re trying to figure out exactly how gender works. These vastly different fields share a common connection — epigenetics.

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In epigenetics, we’re studying how DNA activity can actually radically and permanently change, even though the sequence stays the same. DNA is the long, string-like molecule that winds up inside our cells. There’s so much DNA that it actually gets tangled into these knot-like things — we’ll just call them knots.

So external factors change how those DNA knots are formed. You can think of it like this: inside our cells, there’s different contraptions building things, connecting circuits, doing all the things they need to make life happen.

Here’s one that’s sort of reading the DNA and making RNA. And then this one is carrying a huge sac of neurotransmitters from one end of the brain cell to the other. Don’t they get hazard pay for this kind of work?

This one is an entire molecular factory — some say it’s the secret to life. It’s call the ribosome. I’ve been studying this since 2001. One of the stunning things about our cells is that the components inside them are actually biodegradable. They dissolve, and then they’re rebuilt each day, kind of like a traveling carnival where the rides are taken down and then rebuilt every single day.

A big difference between our cells and the traveling carnival is that in the carnival, there are skilled craftsmen that rebuild the rides each day.

In our cells, there are no such skilled craftsmen, only dumb builder machines that build whatever’s written in the plans, no matter what those plans say. Those plans are the DNA. The instructions for every nook and cranny inside our cells.

If everything in, say, our brain cells dissolves almost every day, then how can the brain remember anything past one day? That’s where DNA comes in. DNA is one of those things that does not dissolve.

But for DNA to remember that something happened, it has to change somehow. We know the change can’t be in the sequence; if it changed sequence all the time, then we might be growing like, a new ear or a new eyeball every single day.

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So, instead it changes shape, and that’s where those DNA knots come in. You can think of them like DNA memory. When something big in our life happens, like a traumatic childhood event, stress hormones flood our brain.

The stress hormones don’t affect the sequence of DNA, but they do change the shape. They affect that part of DNA with the instructions for molecular machines that reduce stress. That piece of DNA gets wound up into a knot, and now the dumb builder machines can’t read the plans they need to build the machines that reduce stress. That’s a mouthful, but it’s what’s happening on the microscale.

On the macroscale, you practically lose the ability to deal with stress, and that’s bad. And that’s how DNA can remember what happens in the past. This is what I think was happening to me when I first started my gender transition, I knew I was a woman on the inside, and I wore women’s clothes on the outside, but everyone saw me as a man in a dress.

I felt like no matter how many things I try, no one would ever really see me as a woman. In science, your credibility is everything, and people were snickering in the hallways, giving me stares, looks of disgust — afraid to be near me.

I remember my first big talk after transition. It was in Italy. I’d given prestigious talks before, but this one, I was terrified. I looked out into the audience, and the whispers started — the stares, the smirks, the chuckles. To this day, I still have social anxiety around my experience eight years ago.

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