They are characterised as X and Y by virtue of their shapes, Doug, when viewed under the microscope.

I have a very simple point to make here, Bruce, which is that this:

"It is apparently normal for people with 2 X chromosomes to be tetrachromat"

...is fake news. The story behind this matter is complex and unresolved. For a start, there's nothing all-or-nothing about it. About half the planet's human population possess two X chromosomes. Females, in other words. At best, a pretty small minority of them are "tetrachromats." That's readily checkable. I'm perfectly happy to concede that you have to have two Xs before you can be one. But it
is untrue to say that it's. Or so to be one