Traits are your beauty mark. They are the unique parts of you, that are seen by others through your feeling-induced gestures, behaviors, or physical attributes. Traits are subconscious and are usually inherited. For instance, if you have a daughter that hums to herself, that trait may remind you of your grandmother who also used to hum to herself.
Mannerisms go along with the creation of your character and are imitations. If you see someone that you want to be like, you may subconsciously choose to imitate some of their gestures and behaviors. For example, children usually pickup many of their mannerisms from their guardians.
Unlike traits, mannerisms can always be changed with a conscious decision.