Recessive or Dominant?
Early-onset Alzheimer's is autosomal dominant: One copy of the altered gene is sufficient to cause it.
Late-onset Alzheimer's is uncertain: Inheriting one copy of the e4 allele increases risk, and two copies even more so, but inheriting it simply increases the RISK of getting the disease, not the disease itself. There are those who have the e4 allele that haven't inherited Alzheimer's, and there are those who have Alzheimer's that don't have the e4 allele: 20 to 25% of those with the e4 allele have Alzheimer's.
Late-onset Alzheimer's is uncertain: Inheriting one copy of the e4 allele increases risk, and two copies even more so, but inheriting it simply increases the RISK of getting the disease, not the disease itself. There are those who have the e4 allele that haven't inherited Alzheimer's, and there are those who have Alzheimer's that don't have the e4 allele: 20 to 25% of those with the e4 allele have Alzheimer's.