X and Y are chromosomes actually and there is Z Chromosome as well. XX is female and XY is male while when Z gets thrown into a human (it's not suppose to be there) serious complications can arise.
Humans don't have a Z chromosome on animals that do it replaces the X chromosome kind of. So female would be ZZ and Male would be WZ
Also I wouldn't be surprised if the next other Gen 6 game will be pokemon Z
Sorry I guess I should make it more clear that I was speaking from a hypothetical standpoint. Humans don't have W chromosomes so if they did somehow get a Z chromosome theoretically that could cause some serious issues. --- I've got a quote that embodies you perfectly, but it's seventy-three posts long, has a few massive flowcharts, and lots of Xion-KMA to Me