The sex chromosomes. Image: Skepchick blog
Post written by Forest Ray, with editorial input by Katharine Heller
We’ve heard it all so many times before: humans with an XY pair of sex chromosomes are male and those with an XX pair are female. Sexual Characters is a series of brief articles on how this is not quite true.
Take Swyer Syndrome, for instance.
The thing about Swyer…
With Swyer Syndrome, a person with the traditionally male XY set of chromosomes develops as a female. In medical literature, Swyer Syndrome individuals are referred to as women. They have functional female genitalia, including a vagina, uterus and fallopian tubes.
Because of this, the syndrome isn’t usually identified until puberty, at which point people with Swyer Syndrome seem to stall out. There’s no period and no development of adult secondary sex features like breasts.
Depending on how badly you crave things like breasts and periods, this might sound more like an evolutionary step forward. Before you go and CRISPR yourself some period-free daughters, though, it’s worth noting that the abnormal puberty is indicative of some downsides.
Most notably, Swyer Syndrome carries with it a relatively high risk of developing tumors in underdeveloped gonadal tissue. These tumors tend to be benign, although can develop into a cancer called a gonadoblastoma.
The “underdeveloped gonadal tissue” is what, in an XX female, would develop into ovaries, so this is basically Nature being a dick, by offering up ovarian cancer to people without ovaries.
If you’re still holding onto the “XY = male” biological fantasy, then you might think that someone without ovaries can’t become pregnant and is therefore really a male, after all.
Think again, sucker.
People with Swyer Syndrome can, in fact, become pregnant and bear perfectly healthy children. Although lacking ovaries, their uteruses are fully functional and capable of providing a nurturing environment to donor eggs.
Causes
But seriously, what causes someone with a Y chromosome to develop as a female?
A variety of genetic variations lead to Swyer Syndrome and although not all of them are known and a general understanding of this syndrome remains incomplete, it often involves a gene called SRY.
SRY is responsible for setting humans on track to become male. Much like your ex-boyfriend, it doesn’t do much beyond that simple task, even when fully functional. If it doesn’t even get off the couch, so to speak, then it won’t even fulfill its one basic function.
Fundamentally, this explains why even people in possession of a Y chromosome need not develop into a “male”. Based on that, feel free to make what you will of the need for the Y chromosome at all.
Other mutations can also cause Swyer Syndrome, although they are incompletely understood. Mutations in a gene called MAP3 Kinase1, or MAP3K1, are also frequently implicated in Swyer Syndrome.
Essentially, MAP3K1 turns on other genes. It’s supposed to do this in a highly selective fashion, but in the presence of certain mutations, it behaves a bit like a dyslexic on Adderall – it’s not getting the right message and won’t stop talking.
But what about the children?
Many cases of Swyer Syndrome arise spontaneously, from random mutations occurring in just the right place and therefore aren’t inherited. It would seem hard to pass Swyer Syndrome on to a child, since it occurs on the Y chromosome and a pregnant woman with Swyer Syndrome is necessarily carrying an egg from someone unlikely to possess the syndrome.
Biology, however, is a big, moist mess.
Cases exist, in which women with Swyer Syndrome caused by SRY mutations have fathers and even brothers with the same mutation on their Y chromosomes. So why did these family members develop as males? IDK, biology sucks. ¯\_(ツ)_/¯ (Seriously, no one knows.)
And, just to complicate things even further, some evidence links women with Swyer Syndrome to a mutation in the NROB1 gene on the X chromosome, meaning that they inherited their syndrome from their mother.
This can happen because for every pair of X chromosomes in a female, one of them has been inactivated. Women with Swyer Syndrome, however, only got one of their mother’s X chromosomes, so if they got one with the wonky NROB1 gene, they also get gonadal tumors an atypical gender makeup.
We are gathered here today…
So if marriage was only to be permitted between “one man and one woman”, then where does that leave people with Swyer Syndrome?
Societies around the globe experience daily tension over issues of gender, with those who view gender as a simple binary affair often pitted against those who view it as a more expansive and multifaceted phenomenon.
For a while, it was hoped that science would provide a definitive answer to the question of whether a person was male or female. Given that it has instead only complicated that question, perhaps we can admit that we were always asking the wrong question?
I would love to hear your thoughts in the comments below.
Note: The title of the series was changed to reflect a more accurate understanding of the difference between “sex” and “gender”.