Dementia: hereditary?

My grandmother was diagnosed with dementia with Lewy bodies about two years ago. Watching the disease progress, I’ve started wondering: to what extent is dementia hereditary? This link states (in reference do DLB)

As far as I know, no-one else on my father’s side has been diagnosed with dementia, but then again, those diagnoses probably wouldn’t go too far back into history anyway. Women on both sides of my family have been traditionally long-lived; when I get old and gray, will I have to worry about dementia in addition to arthritis and excessive hair loss? In general, how much do doctors and scientists know about the hereditary nature of diseases like dementia?

I’m sorry about your grandmother’s illness.

There are genes on chromosome 21 that have been associated with Alzheimers. There are other genes, too, but that is the best known one. The gene was discovered because people with Down’s Syndrome often develop Alzheimer’s in their 40s and 50s-since they have 3 copies of chromosome 21, it was thought that a gene on that chromosome was to blame. This has been backed up by research on Alzheimer’s patients without Down’s Syndrome.

However, it’s not a 100% correlation, you can have the genes and not get Alzheimer’s, you can get Alzheimer’s without the genes.

A family history of lots of long-lived, healthy relatives is probably more important than a history of 1 relative with DLB. Even if your grandmother DID have the gene, you have only a 25% chance of having inherited it from her, and as I said, you can develop dementia whether you have the gene or not, and a significant proportion of people with the gene don’t develop dementia at all.

Basically, don’t let this worry you.