The idea of two sexes is simplistic. Biologists now think there is a wider spectrum than that.
Claire Ainsworth 18 February 2015
As a clinical geneticist, Paul James is accustomed to discussing some of the most delicate issues with his patients. But in early 2010, he found himself having a particularly awkward conversation about sex.
A 46-year-old pregnant woman had visited his clinic at the Royal Melbourne Hospital in Australia to hear the results of an amniocentesis test to screen her baby’s chromosomes for abnormalities. The baby was fine — but follow-up tests had revealed something astonishing about the mother. Her body was built of cells from two individuals, probably from twin embryos that had merged in her own mother’s womb. And there was more. One set of cells carried two X chromosomes, the complement that typically makes a person female; the other had an X and a Y. Halfway through her fifth decade and pregnant with her third child, the woman learned for the first time that a large part of her body was chromosomally male1. “That’s kind of science-fiction material for someone who just came in for an amniocentesis,” says James.
Sex can be much more complicated than it at first seems. According to the simple scenario, the presence or absence of a Y chromosome is what counts: with it, you are male, and without it, you are female. But doctors have long known that some people straddle the boundary — their sex chromosomes say one thing, but their gonads (ovaries or testes) or sexual anatomy say another. Parents of children with these kinds of conditions — known as intersex conditions, or differences or disorders of sex development (DSDs) — often face difficult decisions about whether to bring up their child as a boy or a girl. Some researchers now say that as many as 1 person in 100 has some form of DSD2.
When genetics is taken into consideration, the boundary between the sexes becomes even blurrier. Scientists have identified many of the genes involved in the main forms of DSD, and have uncovered variations in these genes that have subtle effects on a person’s anatomical or physiological sex. What’s more, new technologies in DNA sequencing and cell biology are revealing that almost everyone is, to varying degrees, a patchwork of genetically distinct cells, some with a sex that might not match that of the rest of their body. Some studies even suggest that the sex of each cell drives its behaviour, through a complicated network of molecular interactions. “I think there’s much greater diversity within male or female, and there is certainly an area of overlap where some people can’t easily define themselves within the binary structure,” says John Achermann, who studies sex development and endocrinology at University College London’s Institute of Child Health.
These discoveries do not sit well in a world in which sex is still defined in binary terms. Few legal systems allow for any ambiguity in biological sex, and a […]
Sex isn’t chromosomes: the story of a century of misconceptions about X & Y
The influence of the XX/XY model of chromosomal sex has been profound over the last century, but it’s founded on faulty premises and responsible for encouraging reductive, essentialist thinking. While the scientific world has moved on, its popular appeal remains.
By Ian Steadman
When the International Astronomical Union (IAU) reclassified Pluto from planet to dwarf planet in 2006, it did nothing to change the fact of the existence of Pluto. Its status, however, is an innocuous example of how science is not always an objective descriptor of reality, but an interpreter, loaded with the context of previous generations – how the Greek “planetai” and the post-Copernican “planets” were both labels to describe things that moved in the heavens, even if we realised those things weren’t actually that similar to each other on closer inspection over time.
The scientific process often involves tweaking taxonomies. Humanity saw distant objects above, and the taxonomy we built was simple: two entries, one labelled “planets”, the other “stars”. Over time we added extra things, like asteroids (rocky) and comets (icy), to cover new discoveries – and, then, even further research (and pictures like those returned by the Rosetta probe) meant that some of the things we thought made asteroids and comets very different were really only a reflection of our perspective. (And, for what it’s worth, at the same meeting in 2006 where the IAU created the new term “dwarf planet” for objects like Pluto and “planet” for, y’know, planets, it also voted to use “small Solar System body” for everything else. This too will pass, probably.)
We all believe in the existence of comets and asteroids, even though the colloquial distinction between them makes less and less formal sense – would we bother with two different names if we’d only discovered them today? What purpose would drawing the dividing line between them that way serve?
Famously, when the first taxidermied duck-billed platypus was sent back to London by naturalists working in Australia, it was believed to be a hoax, as it refused to cohere to the then-accepted definitions of mammals and birds by insisting on being a hairy warm-blooded creature that laid eggs. The taxonomical status of the platypus (and the few other egg-laying monotremes that have yet to become extinct) is still a subject of debate to this day – biologists have found it has genes usually only present in fish and amphibians. A male platypus even has ten sex chromosomes (XYXYXYXYXY), instead of the normal two for a mammal.
Ah, but there’s a weasel word there: “normal”. And with sex chromosomes, perceptions of “normal” play a huge role – not only in what we think that they are and do, but in the very existence of the term “sex chromosomes”. This is the subject of Sarah Richardson’s revelatory book Sex Itself: The Search for Male and Female in the Human Genome, a history of the science of sex and the invention of […]
Klinefelter syndrome (KS) is the leading genetic cause of primary hypogonadism and infertility in men.1,2 The clinical phenotype has expanded beyond the original description of infertility, small testes and gynecomastia.3Animal models, epidemiological studies, and clinical research of males with KS throughout the lifespan have allowed us to better characterize the variable phenotype of this condition. This review will provide an overview on what is known of the epidemiology, clinical features, and pathophysiology of KS, followed by a more focused discussion of testicular development and the clinical management of hypogonadism and fertility in men with KS.