Sex determination in humans

This is a six week old embryo. It’s the size of a sweet pea and doesn’t look anything like a human yet. Many things are happening to the embryo at six weeks — its facial features start forming, as do the limb buds that will grow into arms and legs. From the point of conception, the embryo has all of its chromosomes, including the pair of sex chromosomes — in the vast majority of cases either an XX or an XY pair. But at this point, the embryo doesn’t have biological characteristics that are associated with XX or XY chromosomes — that are associated with being biologically female or male.

Instead, the embryo has two bulges, and two sets of ducts, which are capable of developing into either female or male reproductive systems. One of the ducts can become Fallopian tubes and a uterus, while the other duct can become the sperm-producing parts of the male reproductive system. Meanwhile, the two bulges can develop either into ovaries or into testicles. Development associated with sex chromosomes, starts when the embryo is six to seven weeks old, and takes a couple of weeks. What the bulges become, typically depends on the sex chromosomes of the embryo.

Let’s take a look at what happens to an embryo with XX chromosomes! XX chromosomes signal to the bulges to develop into ovaries. They also signal to one set of ducts to develop into Fallopian tubes and a uterus. At the same time, the other set of ducts shrinks and decays. And if an embryo has XY chromosomes?

Well, in that case, the bulges develop into testicles. Testicles then release a chemical messenger, the hormone testosterone. Testosterone instructs one set of neutral ducts to develop into sperm-producing parts. The other set of ducts then shrinks and decays. However, if this set of ducts does not receive instructions from testosterone at the right time, the embryo will develop Fallopian tubes and a uterus.

Which is what normally happens to an embryo with XX chromosomes! After these internal reproductive organs are established, the fetus develops external reproductive organs as well — a vulva or a penis. So the XX and XY chromosome pairs typically determine whether the embryo will develop female or male reproductive organs. But in around 2% of all live births, this is not the case. In such cases, embryos develop reproductive parts that are different to those usually associated with the sex chromosomes they have.

In those cases we talk about differences of sexual development. These differences can happen, for example, because testosterone isn’t released at precisely the right time. Or it can be due to the embryo having an extra copy, or copies, of the X or the Y chromosome. We don’t know that much about differences of sexual development yet. Scientists have a lot more research to do, in order to better help people who might experience problems because of these differences.

Sex determination might seem simple at first, as if it all has to do with just X and Y chromosomes. But it is actually one of the more complicated stages of how we develop as humans. It is a delicate balance of interactions between genes and hormones, and the timing of those interactions. And there is a lot we don’t yet understand.