dementia can be caused by single gene (simple inheritance), or multiple gene variants, or by interactions between those variants with your lifestyle and environment (complex inheritance). If it’s the simple type, and your parents both have the same mutation, you definitely would get it. Complex is less straightforward- you likely did inherit at least 1 copy, but it won’t necessarily mean you will for sure get it. In fact you may not even develop it. It does increase the risk though, and some say healthy diet and healthy lifestyle can reduce this risk to zero.