The Pinnacle Houses

During a significant number of the Empire's later years of the Age of Humanity, the different disciplines of magic, or types, kinds, variations, etc, were streamlined and headed by individual houses. These houses would have the sole right to the usage of their designated or chosen manner of magic. The great and warring House of Fire, one of the Four Elemental Pinnacle Houses, is the most famous example for the numerous conflicts they started, ended and involved themselves in.
Every measure would be taken to produce offspring that wielded the house's brand of magic, even selective breeding through arranged marriages, and the hunting and forceful recruiting of any outside individual who showed capability in that magic.
They were all either destroyed or disbanded in the conflicts that arose when the dragons returned to the world, despite their efforts to stay united.