It'll be interesting to see how TFA deals with the force inheritance, and it obviously will as it's right upfront in the second teaser with Luke's VO. I don't think that line from ROTJ is necessarily saying that it solely comes down to genetics. I think there's still room for everyone to have that potential, but the Force just happens to be stronger than usual in that bloodline. It's a little ambiguous, but so is the nature of the Force, in and out of universe.

Oddly enough while the PT solidified the fact that not everyone has the same potential it also solidified the fact that Force ability is not passed down through generations as Jedi did not have offspring, so youth with Force abilities have been granted such at random and are always born to those (presumably) without Force abilities.