Huntington disease is a hereditary disorder characterized by irregular movements of the limbs and facial muscles, a decline in cognitive ability, and personality changes. In contrast to AD, Huntington disease can be positively diagnosed. Its movement disorders and psychiatric symptoms can be controlled with drugs. However, the progressive dementing nature of the disease cannot be stopped. The cause of AD is unknown.
AD is a multifactorial disease with environmental and genetic components.
Defects in these genes are linked to AD, but AD is not caused by genetics alone. Researchers have been unable to pin down environmental and other factors. The amyloid precursor protein gene is on chromosome 21. A defect in this gene accounts for 1 to 3 percent of AD cases, and this type of AD affects people primarily between 50 and 60 years of age. The presenilin I gene is on chromosome 14.