Nick Lane

You want to switch them off as much as possible, keep them on ice as much as possible.

So very much the differences between how the sexes end up kind of becoming different to each other.

boils down to what are the constraints on your reproductive system.

Yeah.

Well, it does.

Okay.

And the Y chromosome is degenerate.

Yeah, but I mean, there are some things that have lost their Y chromosome altogether.

And they still have sexes because it's not strictly dependent on the Y chromosome.

I mean, again, if you look at what determines sexes across the whole canvas of evolution, it's kind of weird because...

Amphibians, for example, have temperature-dependent sex determination.

So males would develop at a higher temperature than females, or sometimes it's the other way around.

And birds have different sex chromosomes to mammals, for example.

So sex chromosomes have evolved on multiple different occasions.

And what's the Y chromosome doing?

Well, the Y chromosome is basically encoding a growth factor, and that growth factor switches on other growth factors.

And the earliest difference that you could tell between the two sexes in embryonic development is not the activation of the Y chromosome in the SRY gene.

It's actually the growth rate.

And there was a woman at UCL where I am called Ursula Mitfock who spent her career.

She had about 15 nature papers in the 1960s.