Squibs...

[abergoat]

No: HPFGUIDX 160171

Random832 wrote:
> 
> JKR has said that you either are magical or you're not, there's no
> "magical, but not magical enough" middle ground. The only difference
> between a squib and any other muggle is that their parents are
> wizards.

Abergoat adds:

I think Kate's question is great though, because I think JKR has been
clear on the point that a squib requires TWO magical parents. A
magical and non-magical parent mix might be more likely to produce
magical children (per JKR) but it isn't impossible for them to have a
muggle, and that muggle would not be a 'squib'.

So Tobias Snape may have had one magical parent.

Abergoat