Personally, I don't think it matters in the slightest - unless it's a plot point that a character is of mixed parentage, what earthly difference does it make? It may have something to do with 'race paranoia', as it were. There seems to be a tendency among 'minorities' (I put that in quotes because a number of them are actually becoming majorities at this point) to get awfully picky about just how black/asian/chicano/whatever someone is. 'You're not really black - your grandmother was white', or 'you're no chicano - you don't speak Spanish', etc., etc. It's like these things are quantifiable elements, like hops in beer, and if you don't have the exact right amount in you, you don't qualify. I look forward to the point when none of these things will MATTER - I mean, talk about nitpicking.