John and Sarah are contemplating having children, but both have a family history of Tay Sach's Disease, which is lethal usually by the age of 2 or 3 years. John had a brother die of Tay Sach's and Sarah's great grandparents (on her father's side) had a daughter die of the disease. All other members of the family are healthy. What is the probability that John and Sarah's first child will be affected with Tay Sachs?