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Morpheus offers the pills in the MatrixThis is the third part of a series about the complex biological realities of sex. Though the posts build on one another, each can be understood alone.

I recently had the opportunity to teach about the complexities of biological sex in humans (essentially, the content of my post The Truth Behind the Biology of Sex) as a workshop for college-age kids. I was nervous as I prepared to teach this workshop, in a way that I have never felt before while teaching. I felt like I was repaying a debt to the people who taught me valuable lessons about the complex realities of sex and gender, like Kiki, the transgender woman who first opened my eyes to how arbitrary the definitions of male and female really are.

I was also nervous because it wasn’t like the other times I’ve taught STEM topics, where I can just laugh and gently correct misconceptions about evolution or chemical elements. Every ignorant comment about sex, every refusal to learn the complicated truth in favor of comforting patriarchal lies, would fall on me like a blow. When it comes to the biology of sex, it gets personal.

I set the stage with some standard gender justice practices, asking my audience for their preferred names and personal pronouns. Then I said, “In this class, we are going to say words like ‘breasts’ and ‘testes’, so if you’re going to giggle about that, get it out of your system now.” Nobody giggled. But even with that opening, it still took a lot of encouragement to get the kids to talk about sexual anatomy.

I opened with small-group discussions of what it means to be male or female, and wrote their answers on the board. They pointed out the differences between sex and gender, that gender is psychological and sociological while sex is biological. My goal for the class was to point out how psychology and society impose ideologies and categories to the biology that don’t exist naturally.

The students also gave answers like “sex chromosomes.” When I got that answer, I said, “Who here has two X chromosomes? Raise your hand.”

Most of my female students raised their hands.

“How do you know?” I said. “Have you had a karyotype? Has someone looked at your chromosomes under a microscope?”

Blank stares. I said, “Then do you really know for sure? You might change your mind by the end of this class.”

Other answers I got were gonads (ovaries vs. testes) and testosterone levels. “You’re missing something obvious,” I said, though I knew it was because my opening gambit didn’t work as planned.

“Does the person have a penis?” someone said, anonymously, without raising a hand.

“Right,” I said. “External genitalia.” The anonymous wit had hit on something important, though. The question isn’t whether the person has a vulva, or testicles. It’s whether the person has a penis. Male infants born with no penis are usually assigned female; their perfectly functioning testicles are removed and a vulva surgically created (source). Our society has a very phallocentric way of defining sex.

So I talked a bit about the primary and secondary sex characteristics that have been used to determine sex, then asked, “Does anyone see a potential problem with using these characteristics to decide someone’s sex?”

Primary and secondary sex characteristics

Image from the Kallmann Syndrome Organization.

“What about man-boobs?” a student said. A suppressed wave of giggles spread through the class.

“No, he’s right,” I said. “There are males with breasts, and females with facial hair. And the depth of the voice can also vary a lot. Anything else?”

A raised hand. “Genitals can be ambiguous sometimes. In between.”

“Very good,” I said. “Male and female genitals are not polar opposites, but exist on a spectrum.” I showed them this image.

Prader Scale of genitalia

The Prader scale of genital configurations. Image from Texas Children’s Hospital.

“There are many reasons a child might be born with genitals somewhere in the middle of this scale. One example is congenital adrenal hyperplasia. That’s when the adrenal glands produce testosterone in addition to adrenaline. The testosterone masculinizes the genitals of an XX female fetus as it develops.”

Then we moved on to sex chromosomes. I talked a little about their biology, then showed a picture of a karyotype, an image of a person’s full complement of chromosomes, condensed and dyed for easy identification. “Does anyone know what this karyotype is?”

XXY Karyotype

A karyotype (condensed and dyed set of chromosomes) with two X chromosomes and a Y chromosome. Image from University of New South Wales embryology course.

“That’s Klinefelter’s syndrome.”

“Right. Two X chromosomes and a Y. And what body type do you get with Klinefelter’s syndrome?”

“Um. In between?”

“Sometimes. But usually not. Development of breasts and small testes are often seen here, but of course, there are XY males who have the same characteristics. It’s thought that at least 75% of XXY males don’t know their chromosomal status.” The students seemed very surprised by this. See? I thought. I told you you might change your mind about your chromosomes. You might change your mind about a lot of things.

“So the reason why XXY usually leads to a male body type is that the reproductive system of a fetus starts out neutral. About two months into development, if the fetus has a copy of the SRY gene, which is found on the Y chromosome, the gonads will become testes. Without the SRY gene, they become ovaries. So it’s not really the sex chromosomes that determine which way the gonads go, but the SRY gene. So some very interesting things can happen if the SRY gene goes wandering.”

That was when I led the students through an activity about how the SRY gene can become separated from the Y chromosome. When sperm are formed in the testes, there is an opportunity during cell division for the SRY gene to “cross over” from the Y chromosome onto the X chromosome. As shown in the image below, this can result in a sperm carrying a Y chromosome without SRY or an X chromosome with SRY. An XY fetus without SRY will develop into a typical, fertile female and an XX fetus with SRY will develop into a typical, fertile male.

Crossing over in sex chromosomes

Then we had an activity where I gave the students a hypothetical person whose sex they had to determine using some biological indicators I gave them. This hypothetical case study was someone with the SRY gene and Partial Androgen Insensitivity Syndrome, whose receptors can’t fully process testosterone. Such a person has testes, but the body only responds partially to the testosterone released by the testes, resulting in an intersex body type. Then I asked the students to once again discuss what defined whether someone is male or female.

This was my favorite part of the lesson, because I got to circulate around the room and listen to my students’ discussions as they tried to figure all of this out. I loved to hear them apply strong scientific reasoning to the evidence they were given, but I was even more pleased to hear them discuss the sociological implications. I saw them go through all the traits they had given me at the beginning and dismiss them all as too simplistic.

“Testosterone levels have a really broad range from person to person, and anyway, not everyone responds to testosterone in the same way. So how are you supposed to know if that makes you ‘too male’ or not?”

“XY and XX can end up as male or female or anything in between.”

“But if the testes don’t produce anti-Müllerian hormone, then you’ll have testes and a uterus and Fallopian tubes.”

And I think my favorite thing to hear, which I heard in several different variations:

“Wait, but why are there only two sexes? It doesn’t make any sense. This person doesn’t fit into either.”

Most students seemed to agree that such simplified definitions of sex were unfair to people who didn’t fall inside the lines. Most were amazed at the thought that they, too, could be intersex.

Not everyone absorbed the lesson the way I wanted them to. Some people persisted in trying to fit our hypothetical person into a “true sex.” But they were in the minority. And that gave me hope that my generation might be succeeded by a more open-minded one.