Faith doesn't come through a natural genetic process. A person doesn't 'inherit' faith through a natural bloodline; God didn't encode faith into our DNA so it could be passed on to our children. Faith does have a link with blood though -- Jesus' blood. (Romans 3:25) When the disciples asked Jesus to increase their faith (Luke 17:5) it showed that they understood that God, not genetics, is the ultimate source of faith. We couldn't even have any faith but for the fact that he enables us. God calls us and if we answer the call and ask him he will give us the faith that we need. I think we all have doubts sometimes (because our enemy is constantly trying to plant them into our brains). Whenever thoughts of doubt pop into your head, just remember who is putting them there and ask God to help strengthen your faith. He'll do it.