If you chose only one trait when creating the character, you can only exchange it for a new one. You cannot fill the empty slot.
In unpatched Fallout, you could exchange Gifted for another trait and still keep the Character Points.
While minimally useful in Fallout and Fallout 2, Mutate! is a reasonable perk in Fallout Tactics, where it can be used to remove unwanted traits from NPC squad members.
In Fallout Tactics, Mutate! can be used to give NPC party members a trait when they have none already.
The image for this perk is a reference to Marvel Comics' Dr. Bruce Banner/The Hulk. The image of the split personality always appeared at the end of the introscene during the television series, which ran from 1978 until 1982.
Edit