why does simply having XY or XX chromosomes not necessarily dictate sex?


i’ve been out of school for a while, but when i was in school, there was no mention of any SRY gene and it’s functions and ability to travel to either X or Y and implications and all of that. It’s a little confusing for me at the moment so could someone explain?

In: 0

Because the body is complex.

For instance, there is the androgen insensitivity syndrome which means the body is immune to the effects of testosterone. So you are born as an XY, you have testes that never full form or descend because male hormones have no effect on you and since we all start female in the womb, this genetically created male presents as female.


And then there are more than just XX and XY combos. You have what’s called fragile X, that’s it, the girl has 1 X. Then you got the ones where you got 3 instead of 2, XXY, XYY and a few others as well.

As you can see, gender and Sex are very complicated, its not just men and women, its a spectrum of presentations where some are more male then others and some are more female that others. Chromosomes play a role as well as your hormones.

Long story short, when teaching a bunch of 15 year olds biology, they dont go into every detail and exception at a university level.

You were also taught plants need sunlight to survive, yet there are plants that grow in the dark.
You were taught a lot of things that are true in 99% of the cases.

People somehow fail to understand that having a basic understanding of a subject also means you lack some deeper knowledge on the subject.

As for genetics and the XX/XY system.
It’s just a simplification of how things really work.

I’m no biologist.
I couldnt explain it better than that system
It’s a fine system for a large percentage of cases
But it’s not how it works all the time

If you want to fully understand it, you’d need a college level course in genetics and endocrinology. And even then, I wouldn’t expect someone who had these courses to know of every possible exception that can occur naturally.

So as an ELI5: How our body develops depends on a lot of things. At the “visible at certain times with good microscopes” level is chromosome count and length. That’s the level people get to in high school. Scientists who looked closer (aka use analysing methods and computers to visualise things), noticed that chromosomes contain genes and non-coding genom parts. The genes can be “read” and translated into proteins that can have effects on the cell’s functions, its metabolism or it can be released to affect other cells. Some of these are enzymes that change which and how many “sex hormones” are produced, others are the receptors that recognize sex hormones and have an effect on the cell when activated (like on factors that tell the cell how much of another protein to reproduce). Many of the proteins have been identified, but we still don’t the detailed functions of all of them. If you look even closer, the non-coding parts can have effects on how well and how often the coding parts are translated. And then the DNA-strands are rolled up on proteins that can be modified by enzymes, again effecting the rate or ability to read the genes. So the closer you look the more complex it gets.

How our genitalia develop depends on specific genes being present and activated at specific times in our development. And how our brain develops is even more complex. It is influenced by sex hormones, but not as directly as genitalia and it’s not even fully understood by the most knowledgeable scientists in the field.

At any level in this, you can have natural variations (or “mistakes”, but unless they lead to the person not being able to live, I wouldn’t consider it a real mistake). There’s the normal XX and XY; but X, XXY and other variations in chromosome count exist. Then you can have genes translocated onto the “wrong” chromosome. Then there can be mutations in the genes that code for enzymes that influence the hormones, or that code for the hormone receptors, or for other factors that are necessary. These mutations might make no difference, or they make things more effective, or less effective, or completely non-functional. And so on.

In short: Biology is more complex the closer you look at it. Unless you work in the field, you don’t necessarily need to understand every detail, just like you don’t have to be a mechanic to drive a car. But you should listen when people with more background knowledge than you talk about the subject and accept their statements.

Understand that when you’re in school and learning about science, particularly about biology, the content is simplified because of time constraints and the need for additional context. You may have been taught about X/Y sex selection in gross terms because it’s an easy to grasp example in genetics. What they didn’t cover in detail is the details of how the system works (which, with some additional knowledge in genetics would lead you to hypothesize, correctly, that sometimes it doesn’t work).

Anatomical sex is determined through the effects of several genes, particularly one called SRY that is usually found on the Y chromosome. If you studied genetics, you’d know that there are several different things that could knock out the function of this gene, or that could have it break off a Y chromosome and land in an X chromosome so that you can end up with XY people that develop as females, or XX people that develop as males. You’d also be aware that there are chimeric people that are a blend of XX and XY-containing cells that could develop as some combination, and so on.

At the same time, the lesson that there’s a singular determinant factor that behaves in a particular way is still a valuable lesson that gives insight into how the system works, without getting into the weeds of the ways it can sometimes breakdown.