What is the difference between primary and familial Down syndrome? How does each type arise?
Primary Down syndrome is caused by spontaneous, random nondisjunction of chromosome 21, leading to Trisomy 21. Familial DS arises from Robertsonian translocation between chromosome 21 and 14. Translocation carriers do not have DS, but their children have an increased incidence of it. If the translocated chromosome segregates with a normal 21, the gametes will have two copies of chromosome 21 and result in DS