Whether or not an individual develops dementia is powerfully influenced by genes. For Alzheimer's disease, the most common type of dementia, one susceptibility gene with major effects has been identified, but progress finding other susceptibility genes has stalled. Twin studies have revealed that nongenetic risk also plays an important role, as there are many monozygotic twin pairs in which only one individual has dementia. Scientists have argued that gene-by-environment interactions will be key to understanding vulnerability to Alzheimer's disease; but to date, few substantial gene-by-environment interactions have been replicated. Often, too, the nongenetic or lifestyle factor appears to have a protective effect only for those individuals not carrying the risky version of the gene, not for those individuals who are at genetic risk.
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