Jenny Graves

It's not a perfect test, but there is no perfect test.

What makes sex chromosomes so troublesome?

Sex chromosomes are very, very weird.

I mean, all the other chromosomes, 95% of our genome is present in two copies.

And sex chromosomes are present in two copies in females.

There's two X chromosomes, but in males, we've only got one X chromosome.

So there's all sorts of very complicated mechanisms to damp down the second X chromosome in females.

So

X chromosome inactivation is probably the best example of epigenetic silencing of about a thousand genes.

Okay, so for people with two X chromosomes, one of those X chromosomes is...

just sitting there not doing anything.

Yes, that's really where I started back in 1963 was looking at X chromosome inactivation because it seemed like an amazing thing that a whole chromosome is shut off.

It turns out that there are genes on it that aren't shut off and they're probably quite important.

The other thing is, of course, men have only got one copy of the X chromosome and

And that's got lots of genes on it that do all sorts of other things.

It's not a female determining chromosome at all.

There are genes that make colour vision pigments and genes that make blood proteins and genes that make enzymes.

And so they're really necessary genes.

And if something goes wrong, if there's a mutation in one of these genes, and if you're a male, bad luck.

You have a disease or you're colour blind.