Slytherin Purebloods?

[severelysigune]

No: HPFGUIDX 88152

Many people who theorise about the Harry Potter universe and who 
write fan fictions seem to assume that students who get sorted into 
Slytherin must necessarily be purebloods; likewise they assume that 
Snape must be a pureblood, too. But can they be? It seems Slytherin 
House would have to be the smallest of all houses - as far as we know 
from the books, there are less than ten pureblood families left in 
all. I would guess that most wizarding families have Muggles 
somewhere in their ancestry, and that being against 'Mudbloods' does 
not necessarily preclude one's being a pureblood oneself. In the days 
of Salazar Slytherin it might still have been possible to select 
Slytherins on blood basis only, but surely it cannot be the case 
anymore now?