A young family living on a southern Maine farm had just received the devastating news that their 2-year old son, their first-born, is mentally retarded. A geneticist was called in by the parents to find out if there was a history of retardation in their families and to ascertain the probability of having another child with the same handicap. The geneticist assured the parents that since they were so young (both in their 20's) there was little chance of giving birth to a Down Syndrome child (a condition in which mental deficiency is a common trait). Furthermore, the geneticist felt there was a rather remote chance of producing another child with the same birth defect. The parents, satisfied, left the geneticist's office and went home. Later that night they got a frantic call from their cousin who lives across the road from them. It seemed that her daughter was just tested for a learning disability and was diagnosed with a serious mental deficiency. Was the geneticist wrong?

Hint: Do not consider organic (carbon-containing) contaminants or lead.