Well, its not super complex.
Essentially, any breeder can buy a couple nice looking dragons, breed them, and call it his/her own line. The only issue is that many of them just got their dragons from larger breeders. And even then, the largest breeders exchange with one another all the time to keep up diversity.
It is important to note, that after several years of breeding dragons, and not buying a large number more from other breeders, your dragons will eventually wind up being a particular "line". But his based on relatively consistent genetics within your animals.
It is very important to keep careful records or the genetics running through your colony. Good breeders will do this, which makes sure they can offer and maintain the most diverse collection they can. Following their example is vital to success of your animals and their health.
Hope this helped some
phil