Who is Renton's Parents from Trainspotting and what is their importance?

Mark Renton's parents are portrayed in the novel a tragic figures who cannot - try as they might - forgive Mark for his addiction. His mother is a Catholic and his father a Unionist Protestant who encourages his other son Billy to join the military. They had a third son, Davie, who was mentally and physically disabled. His death crushes the family. When Mark Renton overdoses, his parents bring him home to get off heroin there. The final connection between Mark Renton and his parents occurs when Billy is killed in Ulster.