Ipsilateral deviation of tongue in 12th nerve palsy is due to -
Injury to hypoglossal (12th) nerve paralyzes ipsilateral half of the tongue. When the tongue is protruded, its tip deviates towards the paralyzed side because of unopposed action of genioglossus muscle on the normal side of the tongue.
Unaltered action of ipsilateral genioglossus
Unaltered action of contralateral genioglossus
Action of hyoglossus
Action of lateral pterygoid