i'll make an example from my favorite series, Dune. Paul's family was strong with their powers. Paul had the visions, Leto, Ghanima, Alia, all the way to hundreds of generations later and the Bashar Teg. but no where does Herbert suddenly decide while writing the books after half of them are already done to say " the visions are caused by some tiny organism or particle, traced in the blood" that had NEVER BEFORE BEEN MENTIONED. Genetics is one thing. to say his family was strong is another. but i think Lucas made a logical leap that made no sense whatsoever outside his mind. i still say that lucas, who was never the best writer, decided to pull something out of his ass to try and explain this. some of the others have mentioned it. why very little mention in the second movie? in the originals, if the Emperor and Vader know that Force usage can be traced through blood, why not search for samples and find the twins? it doesn't make sense to me, sorry.