Because apostrophes are used for possession as well as contraction, many students get confused about the distinction between it's and its. The solution here is to remember that it's is a contraction, not a possessive: It's = it is. It, like other pronouns （he, his; she, her） doesn't take an apostrophe to show possession:
WRONG RIGHT
The essay's comic tone clashes with it's dark subject.
The essay's comic tone clashes with its dark subject.
Who's and whose have the same distinction:
WRONG RIGHT
Sicinius, who's modern equivalent would be a congressman, accuses the old warrior of being a traitor.
Sicinius, whose modern equivalent would be a congressman, accuses the old warrior of being a traitor.
WRONG RIGHT
The essay's comic tone clashes with it's dark subject.
The essay's comic tone clashes with its dark subject.
Who's and whose have the same distinction:
WRONG RIGHT
Sicinius, who's modern equivalent would be a congressman, accuses the old warrior of being a traitor.
Sicinius, whose modern equivalent would be a congressman, accuses the old warrior of being a traitor.