Nouns ending in -ie are usually feminine, but le génie is masculine.
Génie is one of the rare exceptions: learn it as it is.
Is génie masculine or feminine?
legénieexception
The rule behind it
95.7% of -ie nouns are feminine
In context
Il est possible que Tom soit un génie.
It's possible that Tom is a genius.
Tom est soit un idiot, soit un génie.
Tom is either an idiot or a genius.
Tu as cassé l'ordinateur. Bien joué, génie.
You broke the computer. Nice going, genius.