Regina Barber

Researchers have found more than 450 fish species that can and do change sex. So yes, clownfish, but also gobies and wrasse. It's a pretty long list.

Totally. But this doesn't necessarily translate to humans, right? Like lizards can do a lot of things that we can't and vice versa. We're not fungi. We're not fish. We're different.

Totally. But this doesn't necessarily translate to humans, right? Like lizards can do a lot of things that we can't and vice versa. We're not fungi. We're not fish. We're different.

Totally. But this doesn't necessarily translate to humans, right? Like lizards can do a lot of things that we can't and vice versa. We're not fungi. We're not fish. We're different.

What is it? What's it determined by? And when policymakers refer to sex as this unchanging biological constant, is that reality?

What is it? What's it determined by? And when policymakers refer to sex as this unchanging biological constant, is that reality?

What is it? What's it determined by? And when policymakers refer to sex as this unchanging biological constant, is that reality?

So in humans, sex is determined based on a variety of factors. But for the purposes of this episode, we're going to focus on three of the main ones, chromosomal, chemical and physical.

So in humans, sex is determined based on a variety of factors. But for the purposes of this episode, we're going to focus on three of the main ones, chromosomal, chemical and physical.

So in humans, sex is determined based on a variety of factors. But for the purposes of this episode, we're going to focus on three of the main ones, chromosomal, chemical and physical.

That's true for most humans. Not all. I'll get to that later. But most. And Hannah Clare says that nowadays, when doctors predict fetal sex, usually they're looking at the chromosomes.

That's true for most humans. Not all. I'll get to that later. But most. And Hannah Clare says that nowadays, when doctors predict fetal sex, usually they're looking at the chromosomes.

That's true for most humans. Not all. I'll get to that later. But most. And Hannah Clare says that nowadays, when doctors predict fetal sex, usually they're looking at the chromosomes.

Hannah's a genetic counseling researcher with experience in OBGYN clinics. We're not using her full name here or noting her employer because she's concerned that speaking publicly could hurt her ability to fund her research. But she says this test is super common. Clinicians don't have to wait for the ultrasound to look at the fetus. They just do a little blood test.

Hannah's a genetic counseling researcher with experience in OBGYN clinics. We're not using her full name here or noting her employer because she's concerned that speaking publicly could hurt her ability to fund her research. But she says this test is super common. Clinicians don't have to wait for the ultrasound to look at the fetus. They just do a little blood test.

Hannah's a genetic counseling researcher with experience in OBGYN clinics. We're not using her full name here or noting her employer because she's concerned that speaking publicly could hurt her ability to fund her research. But she says this test is super common. Clinicians don't have to wait for the ultrasound to look at the fetus. They just do a little blood test.

Well, that's really cool. I know, right? So this test tells us the chromosomes that a baby has, but the Y chromosome isn't like an on-off switch for sex. There are sex-influencing genes present in the other 22 pairs of chromosomes too, and there's a lot of variation that's still possible within those genes.

Well, that's really cool. I know, right? So this test tells us the chromosomes that a baby has, but the Y chromosome isn't like an on-off switch for sex. There are sex-influencing genes present in the other 22 pairs of chromosomes too, and there's a lot of variation that's still possible within those genes.

Well, that's really cool. I know, right? So this test tells us the chromosomes that a baby has, but the Y chromosome isn't like an on-off switch for sex. There are sex-influencing genes present in the other 22 pairs of chromosomes too, and there's a lot of variation that's still possible within those genes.

So for a number of reasons, after birth, the baby can develop in a way that's different from what the tests predicted. And that's where this second metric for determining sex comes in.