That last post of mine is not what I thought I'd posted. Please ignore it and read this one (if you must) instead:

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 statement...

"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. Overwhelmingly identifying as 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 "normal." And I'm not going to quibble about gender matters in this thread. If you'd care to delve among my thousands of Mudcat posts you'll discover that I'm actually quite advanced in my thinking about such things and that I lament the fact that we have such a long way to go to reach the understanding and tolerance which would achieve equality for people who don't identify at one of the two extremes of the gender spectrum, to put it clumsily.