This is copied from my Italian grammar book:
"In Italian, the possessive adjective is almost always preceded by the definite article.... The definite article is omitted:
(a) before unmodified words of family relationship in the singular, ie. Mio padre (my father).
(b) after the word "questo" (this) and after numerals.
(c) when the possessive stands alone, ie. È mio (it's mine)"
I know this was a month ago, but I wanted to throw in my bit anyway, and someone else might find it useful :)