Most people who develop Alzheimer's disease do not have a history of the disease in their families. But if you do have a family history of Alzheimer's disease (one or more members of a family have had the disease), then your risk of getting it is higher. When a disease is found in families, the cause could be genetic(heredity), something in the environment, lifestyle choices, or a combination of these things.
A blood test can look for a substance that seems to increase a person's risk for Alzheimer's disease. The gene is called apolipoprotein E-4 (ApoE-4). The presence of ApoE-4 cannot predict for sure whether a person will develop Alzheimer's disease. Many people who have the ApoE-4 gene do not get Alzheimer's disease, and many people who do not have the gene still develop the disease. Most experts do not consider ApoE-4 testing a necessary or useful part of evaluating a person with suspected Alzheimer's disease.
Sometimes people develop Alzheimer's disease at a young age, between the ages of 30 and 60. This is referred to as early-onset Alzheimer's disease or autosomal dominant Alzheimer's disease. It is not common (less than 5 out of 100 cases), and this form of the disease has been linked to defects in specific genes. There is a 50% risk that these genes will be passed on. A person who inherits the genetic defect will most likely develop Alzheimer's disease.