Yingtong Guo, Class of 2022
Many wouldn’t care if they heard that Michigan teenager Hunter Keith has spiky hair, wears baggy pants, likes skateboarding shirtless, and has told everyone he’s always felt like a boy [2]. After all, he was born with the XX chromosome, which means he was assigned – at birth – a girly life and a pink childhood with dresses, fairies, and My Little Pony. Indeed, skeptics of the validity of transgenderism are convinced that biological sex determines gender identity, using this argument to keep transgender adults out of public bathrooms and transgender kids from receiving puberty blockers. However, science has recently shown that biological sex itself can be non-binary and that, therefore, transgenderism cannot be just a cry for attention or a figment of the imagination.
Hunter Keith, 17, Photograph by Lynn Johnson
It’s estimated that 1 person in 100 has some type of disorder of sex development (DSD) – an intersex condition where sex chromosomes and sexual anatomy contradict one another. An XX “woman” with ovaries and a womb, for example, could have two sets of somatic cells, one with the XX chromosome, and another with the XY chromosome {1}, while an XY “man” with undescended testes might have female features and a feminine personality [2]. In these cases, reproductive organs say nothing about an individual’s sex chromosomes in the rest of his or her body, and biological sex remains ambiguous. Since most people with intersex traits still identify as one of two sexes, there must be other significant factors that influence gender identity, including hormones (relative levels of estrogen and testosterone), psychology (self-defined gender identity), and culture (socially-defined gender behaviors) [2].
For the vast majority without DSDs, biological sex is clear at birth, assumed to be rigid and lifelong. However, studies with mice suggest that gonads actually “sway” back and forth between being male and female and require active maintenance. After deactivating the ovarian gene Foxl2 in adult female mice, researchers found, cells supporting egg development supported sperm development instead, while deactivating the testicular gene Dmrt1 in adult male mice achieved the opposite [1]. If female and male sex traits can be switched on and off throughout the lives of other mammals, through a single gene, there is no reason to believe that our biological sex cannot be equally dependent on postnatal stimuli. In this case, to what extent are our eggs and sperm really assigned to us in our infancy – officially and permanently?
So far, we’ve discussed how XX and XY chromosomes can be bad predictors of sexual anatomy in humans and gamete production in (genetically engineered) mice. However, these chromosomes sometimes also fail to “generate” the feminine or masculine brains that their genetic make-up dictates. In particular, an XX “woman” might develop a brain more similar to a man’s, while an XY “man” might develop a brain more similar to a woman’s. Indeed, experiments have demonstrated that transgender people tend to have brain structures corresponding to their gender identity rather than their biological sex even before hormone treatment, with female-to-male subjects showing thinner subcortical areas typical in men and male-to-female subjects showing thinner cortical regions typical in women [3]. Although hidden beneath skulls, our brains are as much part of our human biology as our sex chromosomes and can, therefore, also influence our gender identity – but on a cognitive rather than a sexual level.
So, XX and XY don’t always tell the whole story – the functions of the two chromosomes can be adjusted under certain conditions, and sometimes they can even coexist in a single body. For transgender people, variations in how these chromosomes translate into physical characteristics can be even more pronounced, and understanding them can help us gain insight into why so many adults and kids like Hunter Keith claim that they’re born into the wrong sex despite chromosomal evidence to the contrary.
References:
[1] https://www.scientificamerican.com/article/sex-redefined-the-idea-of-2-sexes-is-overly-simplistic1/
[2] https://www.nationalgeographic.com/magazine/2017/01/how-science-helps-us-understand-gender-identity/
[3] https://www.scientificamerican.com/article/is-there-something-unique-about-the-transgender-brain/
It’s estimated that 1 person in 100 has some type of disorder of sex development (DSD) – an intersex condition where sex chromosomes and sexual anatomy contradict one another. An XX “woman” with ovaries and a womb, for example, could have two sets of somatic cells, one with the XX chromosome, and another with the XY chromosome {1}, while an XY “man” with undescended testes might have female features and a feminine personality [2]. In these cases, reproductive organs say nothing about an individual’s sex chromosomes in the rest of his or her body, and biological sex remains ambiguous. Since most people with intersex traits still identify as one of two sexes, there must be other significant factors that influence gender identity, including hormones (relative levels of estrogen and testosterone), psychology (self-defined gender identity), and culture (socially-defined gender behaviors) [2].
For the vast majority without DSDs, biological sex is clear at birth, assumed to be rigid and lifelong. However, studies with mice suggest that gonads actually “sway” back and forth between being male and female and require active maintenance. After deactivating the ovarian gene Foxl2 in adult female mice, researchers found, cells supporting egg development supported sperm development instead, while deactivating the testicular gene Dmrt1 in adult male mice achieved the opposite [1]. If female and male sex traits can be switched on and off throughout the lives of other mammals, through a single gene, there is no reason to believe that our biological sex cannot be equally dependent on postnatal stimuli. In this case, to what extent are our eggs and sperm really assigned to us in our infancy – officially and permanently?
So far, we’ve discussed how XX and XY chromosomes can be bad predictors of sexual anatomy in humans and gamete production in (genetically engineered) mice. However, these chromosomes sometimes also fail to “generate” the feminine or masculine brains that their genetic make-up dictates. In particular, an XX “woman” might develop a brain more similar to a man’s, while an XY “man” might develop a brain more similar to a woman’s. Indeed, experiments have demonstrated that transgender people tend to have brain structures corresponding to their gender identity rather than their biological sex even before hormone treatment, with female-to-male subjects showing thinner subcortical areas typical in men and male-to-female subjects showing thinner cortical regions typical in women [3]. Although hidden beneath skulls, our brains are as much part of our human biology as our sex chromosomes and can, therefore, also influence our gender identity – but on a cognitive rather than a sexual level.
So, XX and XY don’t always tell the whole story – the functions of the two chromosomes can be adjusted under certain conditions, and sometimes they can even coexist in a single body. For transgender people, variations in how these chromosomes translate into physical characteristics can be even more pronounced, and understanding them can help us gain insight into why so many adults and kids like Hunter Keith claim that they’re born into the wrong sex despite chromosomal evidence to the contrary.
References:
[1] https://www.scientificamerican.com/article/sex-redefined-the-idea-of-2-sexes-is-overly-simplistic1/
[2] https://www.nationalgeographic.com/magazine/2017/01/how-science-helps-us-understand-gender-identity/
[3] https://www.scientificamerican.com/article/is-there-something-unique-about-the-transgender-brain/
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