Family studies of Alzheimer's dementia: results and prospects.

Studies of families located through a proband with dementia of the Alzheimer type have demonstrated transmission of the disorder within families, probably through shared genes. Increasing closeness of genetic relationship and increasing severity of illness are both associated with increasing risk to relatives. Families at high risk are especially valuable for studies of the biology of dementing illness including, in particular, their molecular genetics. Associations at several levels between Down's syndrome and Alzheimer's dementia provide important clues for molecular hypotheses.