Most Alzheimer's is "sporadic". APOE4 is an allele that is a known risk factor for sporadic AD. However, about seven per cent of the Alzheimer population have "Familial" (FAD) or "early onset" Alzheimer's disease. FAD is identical to the sporadic form but it occurs largely due to the inheritance of certain genes which at some point in the family's history "mutated" from having normal to abnormal characteristics. In FAD, this means that if a parent is affected, each child has a 50 per cent chance of inheriting the disease gene and those children will go on to develop Alzheimer's disease in adulthood.
