If anyone can tell us how far we’ve come from Victorian views of sexuality, it’s the makers of women’s undergarments. As we’ve shed our voluminous petticoats and embraced the “man bag,” we’ve also started to accept that human sexuality has a few more shades of gray than may have been acknowledged in days of yore. While it’s not much of a stretch to use clothing to blur gender boundaries, you might be surprised to learn just how delicate your physical gender can be. When it comes to gonads, it really can be the cut of your genes that matters.
And now for the basic biology version of the hated “birds and the bees” discussion: Mom provides the egg (one set of genes), and Dad provides sperm (a second set of genes). When the two meet up, under circumstances best described by parents the world over, they fuse and the two sets of genes form the necessary DNA to make a whole little person. The genes in question are in bundles called chromosomes (23 pairs in human cells), and the sex of the bouncing baby is determined by whether they carry the X or Y flavour of the sex chromosome. As all women have two X chromosomes and men have one X and one Y, the sex of the baby is dependant on whether the lucky egg-meeting sperm is of the X or Y persuasion. So the gender of the baby is determined, forever and always, by the sperm that wins the race, right? Normally, yes, but biology also has some tricks up its sleeve.
The reason that the X and Y chromosomes are so all-important in gender decisions, beyond their nifty alphabetical designations, is the genes they carry. Some of these encode for proteins that produce sex hormones. Hormones are small molecules in your body, and the ones from the ovaries and testes are responsible for all the differences between male and female bodies. Incredibly, every function, feature, and fashion sense that defines our genders boils down to the initial decision for a single cell type to become either Sertoli cells (which are part of the testicle) or ovarian granulosa cells. These cells later play a key role in production of gender-appropriate hormones, resulting in development of everything from genitalia in the womb to scruffy adolescent mustaches.
If the difference between developing Sertoli cells and granulosa cells is the rather important one of gender, surely this process occurs early and is promply cast in stone, right? Actually, like so many things in our body, the cellular decision to develop into one cell type or the other is controlled by a balance of inputs from two opposing “male” and “female” signaling pathways. Even more surprisingly, regardless of the identity of that initial sperm, if key components of one pathway are disrupted, the opposite gender will win out. That is to say that even if someone is genetically male (XY), if there is something wrong with this signaling pathway from the Y chromosome, they will develop like a female, and vise versa. If that gender jack-in-the-box doesn’t knock your unisex socks off, consider that new research indicates this can happen at any point in life. A recent paper describes how disrupting the “male” pathway in adult mouse testes resulted in these organs actually turning into ovaries.These experiments don’t offer practical applications, nor is this at all likely to happen spontaneously (don’t worry guys, you can stop clutching your pants in terror). They do, however, give us fascinating insight into what our genders really are and where they come from. Boy meets Girl, and Sperm meets Egg, but the new life emerging from those unions always looks just like its real Dad: biochemistry.