When someone in the family gets dementia, close relatives worry about whether they will get dementia, too. Genetic risk of dementia is a focus area for researchers. Concerned persons can get a correct overview of the current understanding by checking authoritative, up-to-date sites. This page includes some excerpts and suggested links.
The position paper of Alzheimer’s Society, UK, on genetic testing explains the current understanding of the genetics of Alzheimer’s Disease and other forms of dementia.
Here are a few excerpts from some relevant authoritative pages. These have all been reconfirmed in April 2017:
It is important to understand that genes are only part of the picture. Whatever genes you may have inherited, most people can significantly reduce their chances of getting dementia through simple lifestyle choices. These include not smoking, taking regular physical exercise, eating a healthy diet and drinking alcohol only in moderation (if at all). 
Most people with Alzheimer’s have the late-onset form of the disease, in which symptoms become apparent in the mid-60s and later. The causes of late-onset Alzheimer’s are not yet completely understood, but they likely include a combination of genetic, environmental, and lifestyle factors that affect a person’s risk for developing the disease.
Early-onset Alzheimer’s disease occurs in people age 30 to 60 and represents less than 5 percent of all people with Alzheimer’s. Some cases are caused by an inherited change in one of three genes, resulting in a type known as early-onset familial Alzheimer’s disease, or FAD. For others, the disease appears to develop without any specific, known cause.
Genetic testing is used by researchers conducting clinical trials and by physicians to help diagnose early-onset Alzheimer’s disease. However, genetic testing is not otherwise recommended.
The majority of dementia is not inherited, but this depends very much on the particular cause of dementia. Some (rare) causes of dementia are very clearly ‘inherited’, for example, Huntington’s disease. This is an ‘autosomal dominant’ disease which means that only one faulty copy of the gene is needed in order to inherit the disease. 
You can get genetic counselling to know whether you are at higher than normal risk for dementia. This will help you see whether you should consider genetic testing (this is not available easily in India). One factor is considering whether the results of such testing would make a difference to how you plan your life. More links to understand genetics and related decisions are provided below.
Documents referred to above
-  Position paper of Alzheimer’s Society, UK, on genetic testing: Genetic Testing
-  Alzheimer’s Society, UK: Genetics of dementia
-  National Institute of Aging, USA: Alzheimer’s Disease Genetics Fact Sheet
-  Is dementia hereditary?
- Genetic Testing (from Alzheimer’s Association, USA) (PDF file)
- Genetics of dementia (from Alzheimer’s Society, UK)
- Living with dementia magazine April 2009: Is dementia hereditary? (PDF file)
- Risk factors: Genetics (heridity)(from Alzheimer’s Association, USA)
- Alzheimer’s Disease Genetics Fact Sheet (from National Institute of Aging, USA)
This page is referred to from: Diagnosis, Treatment, Prevention
[The information on this page is a collation from standard, authoritative sources, provided only for convenience. Caregivers can check out these and other sources to get better informed about this topic.]