Muhammad's family and Iranian political dynasties.

I am currently reading the history of the Arab peoples, and of Islam in general, in a book by Professor Albert Hourani. If i am not mistaken,a fraction of Shi'a Muslims, in the matter of the succesion of the Prophet, believe that the "Imam" must belong to the family of the Prophet. Thinking of it, we are actually talking about a dynasty, since what is a dynasty ? Rulling families. Of course the role of the Imam is not purely political, if not at all, but the concept is not very far away. Also, taking  under consideration, the fact the dynasties are the basic political model in Iran for thousands of years ( Achaemenids, Sassanids or Sassanians, Safavids, Qazars, Pahlavi ), can we suppose that one of the reasons why Iranians became mainly Shi'a , a process with depth of time, is because the idea of Muhammad's "dynasty" resembles the Iranian political dynasties?

Well, i did not say that they became Shii on masse, that's why i used the "depth of time phrase".