This is true for only red and green loght detecting proteins (opsins) - the blue opsin gene is on chromosome 7.

The red and green detecting proteins have an interesting history in humans.

Fish, amphibians, lizards and birds have 4 different opsins: for red, green, yellow and blue colours. And the blue opsin sees up into the ultra-violet. Most animals can see waaaay more colours in the world than we (or any mammal) can. So what happened that makes mammal vision so poor?

It's thought that all mammals descend from one or a few species of nocturnal mammal that survived the catastrophe that wiped out the dinosaurs at the end of the Cretaceous. The colour detecting cells (the cones) need a lot of light compared to ones that see in black-and-white (the rods) and therefore nocturnal animals frequently lose cones in favour of the more sensitive rods for better night vision. The mammals that survived the Cretaceous extinction had also lost the green and yellow opsins while keeping red and blue - basically the two different ends of the light spectrum.

Consequently today most mammals still have only 2 opsins so your cat or dog is red-green colourblind.

Why do humans see green? Probably because our monkey forebears, who lived in trees and ate leaves, needed to distinguish red leaves and red fruit (visible to birds) from the green background.

But how did we bring back the green opsin? A whole section of the X chromosome (where the red opsin is coded) got duplicated in a dna copying mistake and then there were two genes for red opsins. As there are different alleles (versions), they could be selected for independently and so one red opsin drifted up the spectrum to be specific for green. So our green opsin is a completely different gene to the green opsin in fish, birds, etc. This kind of evolution happens a lot which is why, for example, there are many families of similar hormones like testosterone and estrogen. And steroids too.

That’s interesting!! I don’t know! We went to the eye doctor and they confirmed (she didn’t totally believe me). She wasn’t in contact with her father and I don’t know if she ever discussed it with her mom or sister.

Another possibility is that she's XY, but the Y never activated, so she developed female but with a single "faulty" X chromosome.

I don't remember my biology classes well enough to say, but wouldn't that also mean that potentially neither of her parents were colorblind, since the Y would've come from her father while the faulty X would've come from her mother? And, if she were XY in this scenario, wouldn't that mean that she'd pass that trait along to her kids as well?