photo tagged 'old family photo' on the internet
A family history of the Disease is linked to greater prevalence of known genetic markers that act as signposts for Alzheimer's in some cases.
Furthermore, recent investigative research asserts that children whose parents have a history of Alzheimer's Disease are more likely to be an APOE4 allele carrier (46% vs 21%, p < 0.001) than offspring without such a parental history.
Interestingly, plasma apoE levels strongly decreased from APOE2 to APOE3 to APOE4 carrying individuals (p < 0.001), resulting in the finding that reduced plasma levels were associated with higher occurrence of Alzheimer's.
They conclude logically that lower plasma levels in middle age is linked to Alzheimer's in old age, based on a study of 400 individuals, 203 active research subjects and 197 control subjects.
Genetics potentially offers another perspective on solving the cognitive decline puzzle that aids in analysis, or triangulation, of whatever data might be available in a particular case or set of cases, micro or macro.