Mistings are allomancers who can use only one of the Allomantic metals. All Mistings in the Final Empire are descended from individuals who ingested lerasium around one thousand years before the empire's fall. The powers granted to these original Mistborn were far more potent and varied. The ability to use allomancy was passed down genetically, but over time the power became diluted. Eventually, Mistings began to appear and full Mistborn became very rare.
Alendi, a Seeker, is the earliest Misting known to us, though Sazed states that other Mistings existed at that time. However, these allomancers had power before the use of the lerasium, and their allomancy seemed to follow different rules. For example, there is no mention of Alendi's using bronze in order to sense the Well of Ascension in his logbook.