I agree with imperialscum, saying, "The Force runs strong in my family" doesn't have to mean it is hereditary. It is like saying, "Faith runs strong in my family" indicating that the family holds to religious values and are strong believers.

Anakin having been a Jedi, and now Luke becoming a Jedi, are enough to warrant the comment about the Force being strong in Luke's family without inferring that it must mean that the ability to use the Force, or how well you can use it, is somehow genetic.

In fact, that seems like a really bizarre inference to make based only on that one line.

Were there other sources for force ability being genetic, or only found in some? It must have been out there somewhere, because the EU used this. Some characters were said to be force sensitive, while others were not. Luke's Jedi Academy would look for and take in Force sensitive students, it is suggested throughout the EU that not everyone has it in them to be able to manipulate the force. Was all of that simply based on Luke's comment about the force being strong in his family? It seems like quite a stretch.