I can't speak as to general racial trends but mixed race marriages are pretty common in my family.

And at a school I once worked at, there were far more children of mixed heritage than purely X or Y heritage. (Interestingly, the biggest racial issues I had to deal with at that school were between kids of X and Y. The mixed raced kids all just seemed to come up as "acceptable" or maybe "enough like me" on their classmates' internal radars.)

But that might've been the school/community I was working in because I've also worked at schools that were nearly all one thing or another with not much diversity at all.

...So I guess it really depends on where you are?