We’ve been underestimating the genetic differences between men and women

Contrary to the well-known (but politically incorrect) nursery rhyme, little girls and little boys are not in fact made of profoundly different substances. Our basic components are subject to the same set of 20,000 genes.

Well, nearly the same set. Our genes are organised into 23 paired structures called chromosomes – and one of those pairs differs between the sexes. Females have two X chromosomes, while men have one X and one Y chromosome. The Y chromosome contains a gene called the SRY gene that leads to the development of the testes, and therefore various other anatomical and physiological features.

Beyond that, the Y chromosome is pretty vacant – containing just 26 other genes, which isn’t much out of a genome of 20,000. Looking at it this way, one could say that men and women are just about genetically identical.

And this appears to support the idea that ‘gender’ – and many of the differences between men and women – are ‘socially constructed’. In other words, if we see more men than women in a certain profession or more women than men opting for a particular lifestyle choice, it’s not biology that’s the determining factor, but society – and society can be changed.

However, as geneticist Jenny Graves explains in a piece for the Conversation, the genetic differences between men and women could be much more significant than the mere numbers would suggest.

“…a recent paper claims that beyond just genes on X and Y, a full third of our genome is behaving very differently in men and women.”

How can this be if, Y-chromosome aside, male and female bodies are working off the same set of genes? The thing to remember is that the genome doesn’t work like an immutable list of discrete instructions. It’s a dynamic text, in which the individual components (the genes) interact with each other and various environmental influences (both within and beyond the body) to express themselves in complex, changing ways. Thus, as well as genetics, we have to contend with epigenetics.

The 27 genes on the Y chromosome could be influencing the expression of a much greater number of  genes. The fact that women have two sets of X chromosome genes could also have a broader effect.

Graves previously suggested that these epigenetic factors could be influencing “a few hundred” additional genes. Now, she writes, this looks like a big underestimate:

“In their new paper, the authors Gershoni and Pietrokovsk looked at how active the same genes are in men and women. They measured the RNA produced by 18,670 genes in 53 different tissues (45 common to both sexes) in 544 adult post mortem donors (357 men and 187 women).

“They found that about one third of these genes (more than 6,500) had very different activities in men and women. Some genes were active in men only or women only. Many genes were far more active in one sex or the other.”

The evidence suggests that practical genetic differences between men and women are much wider than previously thought.

Of course, as human beings we have free will – we are not slaves to our biology. If we want a more equal society then we can have one. But that doesn’t mean that the biological differences between men and women are irrelevant. They do have an influence, and they do go much deeper than the familiar anatomical distinctions.

True equality is about respecting difference, not trying to erase it. And that’s not just a ‘nice thought’ – it can have life and death implications:

“The new study also showed a big difference in expression of a gene previously found to be important for drug metabolism, which could explain why men and women may respond quite differently.

“The Organization for the Study of Sex Differences has campaigned to include women in clinical trials. These results should strengthen their hand.”

To be equal, men and women don’t have to be the same. Which is just as well, because they’re not.