A small addition to the discussion:
If it was Voldy and his Death Eaters doing the judging, I think the following would apply:
muggle + muggle = muggleborn
muggleborn + muggle = muggleborn
halfblood + muggle = muggleborn
halfblood + muggleborn = doubtfully halfblood
halfblood + halfblood = halfblood
BUT
pureblood + muggle = halfblood (or bloodtraitor)
pureblood + halfblood = halfblood
I'm taking this from the fact that Pureblood maniacs actually consider Muggleborns as Muggles (according to the Black family tree where it says that Andromeda married
Muggle Ted Tonks)
Then again, this would be the perfect time to dig out my old post on magical genetics, but unfortunately, I'm not on my home computer, hence can't find it...
