"No, it's a possessive noun, not an adjective."I'm sorry, but a noun in the possessive case frequently functions as an adjective modifying another noun.
In the "old" days, when students were taught to diagram sentences this would not have been an issue. It matters not a wit that it's a possessive noun, it functions as an adjective and that is the point of the question.
Ex:
1. Woe is I.
2. Woe is me.
Pick one. The grammatically correct one is #1, but really. Would anyone say or write that? See what I mean. This rule is an ass, too.
In the case at hand, genius doesn't DO anything. Genius may allow someone to DO something, but genius itself doesn't do ANYTHING. If grammar rules force a person to compose a sentence that says that genius, rather than the person, is doing something, the rule is an ass.
Common sense overrules grammaticians any day of the week.