Family studies in Alzheimer's disease.

The risk for Alzheimer's disease (DAT) increases with increasing genetic relationship to an affected person. The empirical risk is highest for relatives of persons with relatively youthful onset. However, the incidence of cases with late onset may be underestimated because of a ceiling effect imposed by the human life span. Differences in age at onset between index cases (probands) and their affected relatives provide evidence that relative severity of illness is an important parameter of illness to note and analyze. These findings have important implications for genetic counseling. Other evidence from family studies provides leads to molecular genetics. Finally, studies of identical twins, though limited to a few pairs, strongly imply that, in principle, the development of Alzheimer's disease can be prevented.