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 Sep 2018:
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. 
Whether or not we develop a disease can depend on whether we smoke, exercise, have a healthy diet and so on, as well as the genes we were born with and how old we are. This matters because people tend to think of the effects of genes as inevitable or completely fixed, but in most cases this is not true.
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 between a person’s 30s to mid-60s and represents less than 10 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 other cases of early-onset Alzheimer’s, research suggests there may be a genetic component related to factors other than these three genes.
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: Alzheimer’s Society’s view on genetic testing Opens in new window 
-  Alzheimer’s Society, UK: Genetics of dementia Opens in new window 
-  National Institute of Aging, USA: Alzheimer’s Disease Genetics Fact Sheet Opens in new window 
-  Is dementia hereditary? Opens in new window 
- Genetic Testing (from Alzheimer’s Association, USA) (PDF file) Opens in new window 
- Living with dementia magazine April 2009: Is dementia hereditary? (PDF file) Opens in new window 
- Risk factors: Genetics (heridity)(from Alzheimer’s Association, USA) Opens in new window 
- Alzheimer’s Disease Genetics Fact Sheet (from National Institute of Aging, USA) Opens in new window 
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.]