Force Sensitivity is and isn't related to genetics. Yes, Force Sensitives often have a high likelyhood of Force Sensitive Children, but, even Force Sensitives can have a child who isn't (See Satele Shan's son).
Oh, when it comes to the Sith, it's a HUGE shame if you have a Non-sensitive child and you're a Sith, these children are often hidden, or killed.
Force Sensitivity can also randomly pop up in a child who has non-sensitive parents.