It is well accepted that there are a few components to Alzheimer's risk: age (risk increases in old age), environment (severe head trauma, high cholesterol, obesity, or some drugs may increase risk), and genetics. A recent study published in the Archives of Neurology has contributed new evidence towards how genetics may be key. The authors examined more than 100 families, and the incidence of Alzheimer's therein, and found that more than 22% of adult children who had a parent with Alzheimers developed the disease themselves. This risk grew with age--42% of adults 70 and older who had a parent with Alzheimers developed the disease themselves.
What about if there were TWO parents with Alzheimer's? While the authors deferred to a larged sample before drawing conclusions, they agreed that the risk was increased.
Senior author Dr. Thomas Bird of the University of Washington said he was uncomfortable saying the normal risk tripled or quadrupled in people with two affected parents because the study was small and had no comparison group.
"What I'm comfortable saying is that risk is increased and we're working on trying to find out what the magnitude is," Bird said.
Furthermore, the findings may suggest as-yet undiscovered genes other that those already known to increase Alzheimer's risk (the ApoE4 allele), since ApoE4 was correlated with the risk observed in this study but did not explain all the cases of familial Alzheimer's.
Conjugal Alzheimer Disease: Risk in Children When Both Parents Have Alzheimer Disease
Jayadev et al. Arch Neurol.2008; 65: 373-378.
Just as a point of reference (it's in the linked study summary but not listed here):
Prior studies have found a 6 to 13 percent prevalence of the disease in the U.S. population older than 65.
It seems like the risk of developing Alzheimer's Disease goes up quite substantially if one of your parents has it.
The high use of cholesterol lowering drugs seem to have contribute to an increase in Alzheimer's Disease........family genes may also be a factor