Genetics
I do believe that Force-sensitivity is a recessive trait, for Padmé has one recessive Force-sensitivity gene and not being Force-sensitive, and yet giving birth to Force-sensitive twins with only the father being Force-sensitive. No matter who is the biological father of Anakin is (the Force itself is counted as a male person here), he has both Force-sensitivity genes, Shmi only has one (not Force-sensitive either).
In case a non-Force-sensitive person is born from two Force-sensitive parents, there is only one possibility: a mutation within the baby.
If any of you can disprove all of the above (or only one point), may the Force be with you. But I do know that genetics don't influence the power of a person in the Force.