Well I guess my first post should be in reply to someone else's first post. To me it all seems highly random. If you drop a princess on a royal that royal gets traits (I think I did it on accident once and over the next decade he went to inbred).

I'm playing the italians right now and my first heir had an inbred trait. Odd number of toes. I thought I could breed it out so over the course of the game I made sure the heir not only married another factions princess, but never the same faction twice. The lucky girls were from Sicily, Spain, France, England and the Byzantines. I have just hit the high era and what do you know my next heir to be has Odd number of toes.

Explain that one to me. You seem to be trying to do what I am trying not to do and neither of us is succeeding.