I have been thinking about this for a while now and I wanted to see if anyone else could shed some light on this subject.

So far we have seen four Ds: Monkey D. Luffy, Porthogas D. Ace, Marshall D. Teach (Blackbeard) and Gol D. Roger. What is the connection between these people? Obviously they are all pirates and they have somewhat similar facial features (they all have black hair as well). Could they all be related by blood somehow?

We have never seen Luffy's mother, nor his father. Is it possible that Roger is his father? Also when Luffy hears about his grandfather he gets very uncomfortable. Could this have some connection to Roger?

I have not read the manga, so please warn if you are about to reveal some super big spoilers.