Many people will be concerned about whether dementia can be inherited - that is, passed down from an affected relative. People with dementia might be worried that they have inherited it and may pass it on to their children. Family members of people with dementia, such as brothers and sisters, may also be worried that they are more likely to develop dementia themselves.
Genes are the means by which characteristics are passed down through families. They can play a role in the development of dementia, but their effects are complicated and how and whether dementia is passed down - the 'patterns of inheritance' - vary considerably. The information on these pages outlines what we currently know about the genetics of dementia and what it may mean for you.
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).
We all know how children often take after their parents or grandparents. This is in part because some things - physical characteristics, for example - are passed down to us from our parents in the form of about 20,000 different genes.
Genes are the basic units of inheritance. They are made from DNA and are found within almost all the cells of our bodies, packaged in paired structures called chromosomes. In general, everyone has two copies of each gene, one inherited from each parent.
Genes provide the instructions needed to build and maintain our bodies. While much of our DNA is the same for all of us, many genes will differ slightly from person to person. These differences partly account for the physical differences that make each of us unique. They also affect our chances of developing many common diseases.
Genetic variants and mutations
There are two types of differences that can occur in genes. The first are common genetic 'variants'. A variant is not a faulty or abnormal gene. Rather, some genes have multiple different forms (the variants), and people can have different forms. Some are more or less common, but for any of these genes there will be a spread of variants throughout a population.
The role that each gene variant plays in determining any of our characteristics is generally quite small. Most of our individual qualities (eg height, risk of diabetes) reflect the combined effects of many of these variants acting together, as well other factors like our lifestyle or environment. Inheritance of a characteristic that is influenced by a genetic variant is not simple - the inheritance follows a complex pattern.
In contrast, the second type of differences that can occur in genes are rare and are called 'mutations'. The effect of a mutation tends to be greater and can be harmful - a gene with a mutation is a faulty gene. Sometimes a particular characteristic can be traced back to a mutation in a single gene. One example is the gene for Huntington's disease - if an individual inherits a faulty copy of the Huntington's disease gene, they will go on to develop the disease. In these cases, the gene and characteristic are generally inherited in a relatively simple way.
The inheritance of dementia can follow either of these patterns. A few families have a simple inheritance pattern due to single-gene mutations. Many more families have a complex inheritance pattern due to multi- gene variants.
While inheriting dementia directly (through a single-gene mutation) is rare, genes are thought to play some role in almost all cases of dementia. This is because the different genetic variants we all have affect our chance of developing the condition to some degree. Our genetic variants also play a role in determining how healthy we are in other ways, such as our cardiovascular health. This means that they indirectly raise or lower our chances of developing dementia.
Genes are very important in building and maintaining our bodies, but most of a person's physical characteristics and their chances of developing particular diseases also depend a lot on their environment and lifestyle. 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.
Genes and dementia
There are four common types of dementia: Alzheimer's disease, vascular dementia, dementia with Lewy bodies (DLB) and frontotemporal dementia (FTD). How important genes are in these different dementias varies considerably. For example, the role of genes in FTD seems to be much greater than in vascular dementia.
These pages give an overview of the role of genetics in these different dementias.