For Andrew Sinclair, studying sex determination has been his whole career. In 1990, he was working as a postdoc at the Imperial Cancer Research Fund in London in the research group of acclaimed geneticist Peter Goodfellow. The lab was on the hunt for the male sex-determining gene.
“I had been walking along the human Y chromosome and trying to get closer and closer to it,” said Sinclair, who is now the deputy director of the Murdoch Children’s Research Institute in Melbourne.
That “it” ended up being SRY, or sex-determining region Y gene, the specific part of the Y-chromosome that drives human testis development in an embryo. Humans have 23 pairs of chromosomes, which are bundles of DNA that contain all the information necessary for cell function and life. The 23rd pair of chromosomes are referred to as the sex chromosomes. Generally, cells in female, referring to sex, organisms have two X chromosomes, and cells in male organisms have an X and a Y.
Goodfellow’s team had now found the part of the Y chromosome responsible for producing an organism with male sex organs.
“It was quite a moment,” Sinclair said.
Sinclair had analyzed DNA from individuals with XX chromosomes but who had developed as male, finding that small portion of the Y chromosome, SRY, had attached to the X chromosomes, enabling testis development. He then looked at Y chromosome DNA from several mammalian species and found that the SRY gene was present in them all, suggesting that it had an important role in male sex determination.
Crucial to confirming this hypothesis in humans was DNA from individuals who had XY chromosomes but developed female sex organs. Sinclair and his collaborators analyzed Y chromosome DNA from 50 XY males to that of the two XY females by making several copies of the genetic material, isolating the SRY gene and sequencing it.
The team found that across the 50 male samples, all the SRY sequences were the same, but those of the XY female patients contained tiny variations. Genes code for proteins, and these miniscule differences in the SRY genetic code resulted in a protein with just one different amino acid. Amino acids are protein building blocks, and this single amino acid difference impacted SRY’s ability to drive male development in a fetus. Subsequent experiments showed that injecting just the SRY gene into XX mouse embryos caused them to develop into males. Thus, the scientists concluded that SRY was both necessary and sufficient on its own for driving typical testis development.
“That was sort of ‘Wow, we got it!” Sinclair said.
Since then, Sinclair and other developmental biologists have been searching for more genes that influence how the sex organs, or gonads, develop. Unlike what many think, this process is not binary.
“We like to be in a black box and a white box, in a male box and a female box, and we struggle with the sort of the grey zone. But biology works on a spectrum and it is all grey zone,” Sinclair said.
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Sex – an individual’s particular chromosomal makeup, anatomy and hormonal profile –– is also a spectrum. Conditions like those of the two XY females in Goodfellow’s study are part of a broad category of medical variances called “differences of sex development,” or DSD. DSD describes traits associated with atypical sexual development where someone’s body may not fit the typical male or female norm.
DSD looks different for everyone. Individuals with DSD, many of whom also choose to use the term intersex, may present with indiscriminate genitalia or have both ovarian and testicular tissue despite having either XX or XY chromosomes.
The variety of DSD diagnoses means that patients and families can discover conditions as early as birth or as late as when trying to conceive. Prenatal screening can catch some conditions like Klinefelter’s syndrome, where males are born with an extra X chromosome and experience developmental delays and low testosterone production. Ambiguous genitalia are usually detectable at birth and can sometimes be a cause for concern for congenital adrenal hyperplasia, where the adrenal glands, walnut-sized organs that rest above the kidneys, are not able to balance the production of essential hormones.
Sometimes individuals don’t find out until their teen years when they notice that puberty isn’t making sense or when they experience fertility struggles later in life due to incomplete gonadal tissue or missing reproductive organs.
“From personal experience, that’s really hard to find out,” said Adam Davies, health educator at Lurie Children’s Hospital’s Supportive Program for a Range of Urogenital Traits, or SPROUT, clinic in Chicago. Davies has a DSD that prevented their gonads from developing at puberty.
In addition to possible health consequences, individuals with DSD also face stigma, and families often lack support and guidance to navigate their child’s diagnosis.
Noi Liang, an intersex patient advocate at Children’s Hospital Colorado, knows this firsthand. She has complete androgen insensitivity syndrome, meaning she has XY chromosomes but her body can’t recognize androgens, which are a type of sex hormones primarily responsible for male sex characteristics, and thus never fully developed male gonads.
Liang was born with undescended testes, meaning that they were in her abdomen, and no ovaries, but she did not know at the time. By the time she was 17, she had still never had her first period. Her gynecologist ordered chromosomal tests and abdominal imaging, sharing the results and DSD diagnosis with only her mother.
Liang underwent abdominal surgery being told it was to remove potentially cancerous ovaries. It wasn’t until Liang was in her 30s and combing through old medical records that she discovered that they had hidden her diagnosis from her. The surgery was to remove the undescended testes.
“It was very emotionally difficult to find out that I’d been lied to,” Liang said. “For essentially, I don’t know, 15 years of my life, I didn’t have the basic information about my own body that I needed to actually physically care for myself properly.” For example, Liang found that the estrogen supplements her gynecologist had prescribed were too low for her true condition, which ultimately impacted her bone health.
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Misunderstanding around sex and the science behind it is ubiquitous, and new legislation will make it worse.
“When we look at federal policy, I’m concerned that that will happen,” Davies said.
The U.S. federal government’s recent gender ideology executive order says it will only recognize two sexes, male or female, leaving intersex people unrecognized. The crackdown on NIH-funded sex and gender research has already impacted DSD work. Threats to remove federal funding for institutions that do not heed this order also affects access to clinical care.
Individuals and families affected by DSD have questions, especially as scientific research and clinical management of these conditions are rapidly evolving. Grounded in conversations with experts, this blog aims to investigate and answer those very questions.
This blog will dive into the scientific advancements that hope to help those affected by DSD better understand and manage their conditions. This includes gonadal tissue cryopreservation, or freezing, options for patients whose gonads are removed for a medical reason, such as an increased cancer risk that cannot be adequately monitored. Preserved tissue could be used for potential future fertility and hormone restoration. Relevant research also includes modeling embryonic development in the lab to identify genes crucial for sex development, which can help genetic testing and improve DSD diagnostics. This allows doctors to better understand an individual’s condition and make a care plan.
“My job is to understand how this person's body works, to help them understand it and to help them live a happy, healthy life,” said a pediatric endocrinologist specializing in DSD care who chose to stay anonymous given current volatility around this subject.
Despite legislative limbo, experts are optimistic that this research will also improve public understanding of sex as a spectrum.
“I think the more we can do the more we're going to be able to educate people and make them realize that there is much more complexity out there than just the two sexes that we think about,” Sinclair said.