Phineas' actual father has not yet been released to the public, it is a possiblity not even the writers know who it is yet because they might not have given it any thought. Well, me being the over actively imaginative person I am, have given thought to that too. Trying relate the Flynn's to other characters in the series... the closest thing I have ever gotten to would be actually Dr. Doofensmhirtz (I have thought out multipule scenarios to prove myself possibly correct eg.phineas makes inventions and so does Doofensmhirtz). But it has never been mentioned and most likely I'm wrong because well, I usually am. But it would seriously be the most epic thing in all of cartoon history. I bet they'd have to make a whole movie to conclude the series (hoping it's atleast two years from now). And r