Meiosis...and I am waaaay tired...
It seems to be evolutionarily advantaged to have two (major) sexes for reproductive purposes...and fixing those sexes genetically.
...I did not realize there is such a thing as ZW sex determination genes...learn something new every day...
your chromosomes dictate all of your physical attributes. If they were the same...we'd all be the same. At least the men...as they are the only ones with Y's.(that's if I follow your question.)
I had mine altered to a K in memory of my dear old mum. No hang on, that was a tattoo. Whats the question? Crumbly wotnow?
Y, do you ask?