Sex not as simple as X and Y

| February 29, 2016
sex chromosomes
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Individual stories of transgender transformation, like those of Caitlyn Jenner, former tennis player Rene Richards, and other individuals have shown us that gender identity can be fluid. It has been generally assumed that sex differences — maleness and femaleness — are more fixed. But now that belief is under challenge.

The Y chromosome triggers embryonic development as a male. For years it was assumed that was it’s sole function. That was it.

“The Y chromosome was essentially written off as the runt of the human genome,” said Dr. David Page, a geneticist at the Massachusetts Institute of Technology, “as a sort of genetic wasteland that didn’t really merit anyone’s serious attention.”

But those assumptions were too simplistic. Recent studies have shown that not only is the Y chromosome here to stay, it also does other things besides confer maleness.

“The old textbook description says that once maleness is determined by a few Y chromosome genes and you have gonads, all other sex differences stem from there,” said geneticist Andrew Clark of Cornell University, who was not involved in either study. “[New research] opens up the door to a much richer and more complex way to think about the Y chromosome.”

Among those differences, say geneticists, is a more fluid understanding of what it means to be male or female. Today’s gene sequencing and other techniques have revealed a more complex picture of what eventually makes us male or female:

  • A woman receiving an amniocentesis in an Australian hospital was told she had a perfectly normal baby, but that some of her cells carried two X chromosomes (as expected), but another set of her cells had an X and a Y chromosome.
  • Indian surgeons operating on a 70-year-old man for a hernia discovered that the man had been born with a uterus and fallopian tubes (which didn’t seem to interfere with his having fathered four children).

While these stories may sound like something out of a supermarket tabloid or “News of the Weird,” they are part of a syndrome called Disorders of Sex Development (DSD), and they are rather prevalent — about 1 percent of the world’s population has some kind of DSD. That’s not rare.

Shifting sexes

Some DSDs, such as trisomies like XXY or XYY have been known for some time. But in other DSDs, gonadal cells (ovaries or testes) may be one sex but the rest of the person’s body another. Some scientists have looked for abnormal development for the source of confusion. Others, however, have found evidence that cells may shift between male and female later in life. Turning off some ovarian genes in adult mice, for example, may prod cells that normally produce eggs to produce sperm instead. Inactivating another gene, meanwhile, had the opposite effect (turning from making sperm to making eggs). Still other studies found that women may have fetal cell sin their blood for decades after giving birth, and that a mother’s cells can stay in the body in her children until adulthood.

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A lot of answers may lay in the makeup of the Y chromosome. But not much has been discovered about this small but influential structure. In 1990, scientists discovered a gene, called SRY, which appeared to determine male development on the Y chromosome. If you had SRY, you were male. That was it.

But the Y chromosome is unusually difficult to study, and that fact has delayed many discoveries of how the Y chromosome works. Much of the Y is made up of repeating, or palindromic (reading the same backward as forwards) sequences of DNA. Using older methods of restriction enzymes and DNA digests meant studying a chromosome in discrete packets. But when most of the pieces of DNA are identical, it was nearly impossible to rebuild the chromosome and understand what these DNA repeat sequences may do.

Today, scientists are getting an upper hand, thanks to next generation sequencing techniques. Alexander Godfrey, a graduate student at MIT, and his laboratory director, longtime sex chromosome expert David Page, are starting to shine new light on how the Y chromosome exists, and why its role remains important, if not quite as distinct as previously thought.

And no time too soon. Many countries and cultures decide roles in their societies based solely on whether one is female or male. And on more specific issues, medical doctors are now faced with thornier dilemmas in dealing with young patients with a DSD. Do you automatically perform “corrective” surgery on a newborn patient with evidence of both gonads (a lawsuit filed in South Carolina is contesting this practice)? What do you tell the parents and the children, and when do you tell them?

Andrew Porterfield is a writer, editor and communications consultant for academic institutions, companies and non-profits in the life sciences. He is based in Camarillo, California. Follow @AMPorterfield on Twitter.

The GLP featured this article to reflect the diversity of news, opinion and analysis. The viewpoint is the author’s own. The GLP’s goal is to stimulate constructive discourse on challenging science issues.

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