Question: How does family history affect the risk of developing Alzheimer's disease?
Answer: There are two ways in which family history controls the risk for Alzheimer's disease. Simply having a parent or a brother or sister with Alzheimer's increases the risk by about double. There are also identified genetic factors, some of which we can test for, that increase the risk for Alzheimer's. Some of these actually cause the disease -- these are rare forms, in which Alzheimer's begins very early, before age 60. And these are passed from one generation to the next, and always cause disease.
In addition to these causal genes, there are risk factor genes that we can test for. However, these risk factor genes are complex in that you can have the risk factor and still escape Alzheimer's disease, or you can not have the risk factor but still get the disease.