What determines whether a muggle born will happen?

[Elizabeth Snape]

No: HPFGUIDX 161417

 

 
Marion here:
> 
> This has always been an inconsistancy that bothers me a bit.  The 
emphasis is on "blood" but you've got Hermione, for example, who has 
no other wizards in the family, Harry, who's considered "pure blood" 
but whose maternal grandparents are probably muggles, and Filch, who 
comes from a wizard family but is a squib.  So my contention is that 
blood has nothing much to do with it.  And in that case, how can 
there be so much discrimination based on "blood"?  
> 

Snape's Witch replies:
No, Harry isn't considered  a pureblood.  Harry's a half-blood just 
like Tom Riddle and Snape. IMO that's deliberate similarity on Jo's 
part. True, both of his parents are magical but that's not the same 
as being pureblood.

Snape's Witch