Well midi-chlorians go back to the third draft of Star Wars really. In that one Obi Wan says that people are born with lots of the Force stored within them, but then laments that he might not be up to the challenge of rescuing the princess because he is old and has little of the Force left in him. Lucas cut this part out for the final draft, but with ROTJ he brought back the whole physical connection, stating that Luke is strong with the Force because he is Anakin's son, and Leia strong because she is his daughter--in other words genetics, biology. I think ROTJ brushed off this whole "sister" plot point so quickly that most people weren't even aware of the transformation that Force underwent with that plot point.