Age of the Dragons
Dragonkin, along with Orcs, are a true race that did not originate from the creators. They were bred from human stock by the dragons as the great winter approached. Each dragon has a clan of kin in his or her service. Sometimes members of other races will join these clans as well, but they are never truly considered dragonkin (nor is there any sort of ritual to change their race – it is genetic). Dragonkin carry some of the heat of the dragon in their blood, allowing them comfort in a wider degree of temperatures. They also have the ability to see heat, and are of sturdy stock.
Members of a Dragonkin horde would not likely be suitable for play (unless perhaps a Dragon or his Drake deal with people differently in a locale), but a defector or exile would be acceptable.
As Dragonborn in the PHB.
While Dragons in this game come in all different colors of scales, they are all treated as Red Dragons. Draconic Ancestery must therefor be Red.