The two basic types of AD are familial Alzheimer’s disease (FAD) and sporadic (seemingly random) AD. FAD is a rare form of AD, affecting less than 10 percent of all AD patients. It is associated with gene mutations on chromosomes 1, 14, and 21. FAD is the result of a certain dominant pattern of inheritance for one of the above chromosomes, and therefore is inherited. It is marked by earlier onset (usually before age 60) than the sporadic form of AD.
Diseases such as cystic fibrosis, muscular dystrophy, and Huntington’s disease are single-gene disorders. If someone inherits the gene that causes one of these disorders, that person surely will get the disease, unless it is prevented or cured by other means. On the other hand, AD is thought to involve several genes. For example, although the genes for FAD can be passed on, FAD is not a single-gene disorder. More than one gene mutation can cause AD, and genes on multiple chromosomes may be involved in some cases.
Source: Connection Magazine [Volume 5(1), Winter/Spring 1996]