Most personal pronouns take a final ‑s to indicate possession: only the first‐ and second‐person singular forms are exempt. All have the ‑s, and, because they are personal, do not take an apostrophe to indicate possession.

The word its can be a personal pronoun (in the genitive case — but not in the nominative or objective cases) or a personal adjective. Both indicate possession, but the former is a substantive and the latter is not. Personal pronouns and personal adjectives never take an apostrophe to indicate possession.

• The personal pronouns in the genitive/possessive case are mine, thine, his, hers, its, ours, yours, and theirs.
• The personal adjectives are my, thy, his, her, its, our, your, and their.

The relative pronoun whose nominative case is who and whose objective case is whom has a genitive case of whose.

The impersonal pronoun one, not being a personal pronoun, takes an apostrophe to form one’s. The corresponding impersonal adjective is identical in form to the impersonal pronoun just mentioned: one’s.