image by: Andy Blackledge

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Nettie Stevens discovered XY sex chromosomes. She didn't get credit because she had two X’s.

At the turn of the 20th century, biologist Nettie Stevens was driven to solve a scientific mystery that had perplexed humanity for millennia. The mystery was so simple but daunting: Why do boys become boys and girls become girls? Her sex chromosome discovery in 1905 “was the culmination of more than two thousand years of speculation and experiment on how an animal, plant, or human becomes male or female,” historian Stephen Brush explains in The History of Science Society. “At the same time it provided an important confirmation for the recently revived Mendelian genetics that was to become a central part of modern biology.”

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Articles of Interest

Beyond Just XX or XY: The Complexities of Biological Sex

One of the reasons human biology is so interesting is because it’s so complex; rarely are there neat answers. We’re often taught that when it comes to biological sex, everything fits into two boxes: XY and XX. However, it turns out that, just like the rest of biology, things are a little more complicated than that.

Beyond X & Y: Chromosomes and Sex Organs

When it comes to sex chromosomes, there's more to it than XX or XY.

How do the X and Y chromosomes differ and how does that contribute to sex determination?

You're right that it just isn't having an X and a Y chromosome that makes a male (or that having two X's makes you a female). It is what our cells do with these chromosomes that makes an embryo develop as male or female. Specifically, there is a gene on the Y chromosome called SRY that makes a big difference. This gene tells the embryo to develop male features. Without the signals from the SRY gene, embryos develop into females.

More Women Than Expected Are Genetically Men

Sex chromosomes usually determine whether you are female or male. Women are XX. Men are XY. However, genetically, a few women are actually men. They grow up as women with a woman’s body, and most only discover well into puberty that they are different. Danish researchers map for the first time how many women are genetically men. The proportion was higher than expected.

Swyer syndrome

People with Swyer syndrome, however, have an XY genetic makeup, or genotype, but develop a vagina, uterus, and Fallopian tubes instead of a penis and scrotum. This is referred to as a sex reversal, which is also evident in their overall physical appearance, also refered to as phenotype, resulting in unambiguous female external genitalia.

Swyer Syndrome: A Look into Women with a Y-Chromosome

Swyer Syndrome refers to a chromosomal disorder in which a genetic male has a dysfunctional Y-chromosome. Specifically, the parts of the Y-chromosome which control the development of male genitalia are unable to function. As explained before, all fetuses will be female by default, unless a Y-chromosome is present. If the Y-chromosome is unable to actually alter the development of the fetus, however, the fetus will remain a female despite having a Y-chromosome.

The Tenuous Beginnings of Men

Daughters inherit many things from their fathers, but a select few get something unusual—a Y chromosome. Women typically have two X chromosomes while men have an X and a Y, but some XY people are born with female genitals and a uterus. They’re almost always raised as girls from birth, and their hidden Y chromosome only becomes obvious during puberty. That’s because they don’t develop working ovaries, and without these organs providing a flood of hormones, they don’t menstruate, grow body hair, or develop larger breasts on their own. They’re also sterile. This condition, known as Swyer syndrome, is often caused by changes to SRY, a gene on the Y chromosome that acts as a master switch for maleness.

Women with XY Chromosomes and the Debate in Sports

Understanding women with XY chromosomes: Can they have children or compete in sports with women who have XX chromosomes? How do they differ from transgender individuals?

X and Y

As the noted scholars Salt N Pepa once said – “let’s talk about sex [chromosomes]”. Everybody’s got them – those of us born female have two X chromosomes, while males have one X and one Y chromosome. There’s also a minority of people who are born with different numbers and combinations of sex chromosomes, and who might consider themselves female, male or intersex, although we won’t go into more depth on that here (and, of course, your biological sex does not determine your gender identity – plenty of people who identify as men have XX and as women have XY and frankly that’s no-one’s business but theirs).

X&Y Chromosome Variations

Males usually have one X and one Y chromosome, and females usually have two X chromosomes. Occasionally, because of problems with the formation of a parent’s sperm or egg, a child can be born with extra or too few sex chromosomes.

XY Chromosome

There is speculation that those two boxers who had been disqualified from IBA fights due to failed gender tests—and masculine appearance—is they may have a disorder called Swyer syndrome. This isn’t a fact, but it would explain why they would have been declared women at birth and always identified as women. Those with this very rare condition have a male Y chromosome despite their female sexual hardware and offer no male advantage.

XY Chromosomes Only

As I waited in anticipation for my first and second, I am now waiting to hold my next special addition to the family: my third son. In the meantime, I am posting all of my Barbies and American Girl dolls on eBay and treasuring the fact that everyone in my house thinks they are stronger than me.