How do muggles get admitted to Hogwarts?

[Robert Jones]

No: HPFGUIDX 79823

"Anna" writes that JKR said somewhere that when a magical child is 
born his or her name is written down in a book by an enchanted quill 
and when the child turns 11 he or she gets a letter from Hogwarts.  
I think I have read that too.  But this raises the question of how 
does the quill know what muggle is going to be a witch or wizard?  
How did the quill know the day Hermione was born that she could be a 
witch?  Can muggles learn to be a wizard or witch, or must they be 
born that way?  Are some muggles born with a magic gene?  Does this 
mean Lilly and Hermione were really witches from birth, although 
they had muggle parents?
Boris (bewildered as always)