Sex is more complicated than just X and Y

If you've got two X chromosomes you're a woman, if you've got an X and a Y you're a man, and that's all there is to it ... if I had a dollar for every time I'd heard this, I'd be kicking back relaxing in a million-dollar mansion somewhere.

Sex and gender are much more complicated than just XX or XY.

Even if we just look at biological sex, things are not as straightforward as most people believe.

Yes, in humans, biological sex is determined by our sex chromosomes.

Yes, in humans, the majority of people who have two X chromosomes develop female characteristics, and the majority of people with an X and a Y chromosome develop male characteristics.

But it isn't always the case.

Whether we develop male or female characteristics mostly comes down to the presence, or absence, of a gene on the Y chromosome, called the sex-determining region Y (SRY) gene.

Early in embryonic development, the protein produced from this gene initiates the development of testes, and prevents the development of ovaries. In the absence of this protein, female structures will develop.

This is why most people who have two X chromosomes develop as females, and most who have an X and a Y develop as males.

But what happens if the SRY gene is present on the Y chromosome, but the gene is non-functional? Or if there is a Y chromosome that is missing this SRY gene?

In these cases an embryo, despite having a Y chromosome, will develop female characteristics.

So yes, there are females who have an X and a Y chromosome.

This then raises the question, can the opposite happen? Can someone with two X chromosomes ever be male? The answer, again, is yes.

Some people inherit an abnormal X chromosome, which has a copy of the SRY gene. If this happens, they will develop male characteristics, despite lacking a Y chromosome. Then there are other cases where the SRY is only partially functional, leading people to develop ambiguous genitalia.

Not to mention cases where people have unusual numbers of sex chromosomes, like two X chromosomes as well as a Y, or just one X chromosome.

Sex, gender and sexuality can be confusing to navigate. But let's move beyond the old XX = female, XY = male debate. Biology, just like is people, is much more diverse than that.

Dr Mary McMillan is a lecturer at the School of Science and Technology, University of New England