How is Alzheimer Disease inherited?
Around 75% of the time, Alzheimer Disease (AD) is not inherited. 25% of the time Alzheimer's disease can be inherited. This is called familial AD. There are at least four known genes that can cause familial, or inherited Alzheimer Disease.
We all have two copies of every gene, including two of the PSEN1 and PSEN2 genes that can cause familial AD. We inherit half of our genetic information from our mom and half from our dad; so one copy of each gene is inherited from our mother and the other from our father. If a parent has familial AD caused by PSEN1 or PSEN2, there is a 50% chance that any of their children will also have AD. This is called autosomal dominant inheritance. Only one copy of the gene needs to be not working (mutated) to lead to symptoms of Alzheimer Disease.
We also have two copies of the APP gene. One of our APP genes is inherited from our mother, and the other is inherited from our father. Sometimes AD caused by APP gene changes is inherited in a dominant fashion like PSEN1 and PSEN2.
Other times, both copies of the APP gene must have genetic changes (mutations) for someone to have symptoms of AD. In this case, if both parents have 1 APP gene mutation, there is a 25% that any child will have symptoms of AD. There is also a 25% chance that any child will inherit neither parent's APP mutation. There is a 50% chance that any child will only inherit one APP gene mutation and be a "carrier" like their parents. Someone with AD caused by APP gene mutations cannot have a child with Alzheimer Disease unless their partner also has at least one APP gene mutation.