
Originally Posted by
jkrowling's official site
A Squib is almost the opposite of a Muggle-born wizard; he is a non-magical person born to at least one magical parent. Squibs are rare; magic is dominant and the resilient gene.
Squibs would not be able to attend Hogwarts as students. They are often doomed to a rather sad kind of half-life (yes, you should be feeling sorry for Filch), as their parentage often means that they will be exposed to, if not immersed in, the wizarding community, but they can never really join it.
Sometimes they find a way to fit in; Filch has carved himself a niche at Hogwarts and Arabella Figg operates as Dumbledore's liaison between the magical and Muggle worlds. Neither of these characters can perform magic (Filch's Kwikspell course never worked) but they still function within the wizarding world because they have access to certain magical objects and creatures that can help them (Arabella Figg does a roaring trade in cross-bred cats and Kneazles ...) Incidentally, Arabella Figg never saw the Dementors that attacked Harry and Dudley, but she had wnough magical knowledge to identify correctly the sensations they created in the alleyway.