If Harry Potter's mother was a witch, how could Petunia Dursley be a mgugle?

If you read the books, you would know-your husband and daughter should too, since they are reading the book. Harry Potter's mother, Lily, is what the wizarding world would call a muggle-born ( witch or wizard with muggle (non-magic people) parents. Harry's friend Hermione is also a muggle born. Petunia is just a plain old muggle like her parents.