Huntington's disease
Huntington's disease, chorea, or disorder (HD), is a neurodegenerative genetic disorder
Genetic disorder
A genetic disorder is an illness caused by abnormalities in genes or chromosomes, especially a condition that is present from before birth. Most genetic disorders are quite rare and affect one person in every several thousands or millions....

 that affects muscle coordination and leads to cognitive decline and dementia
Dementia is a serious loss of cognitive ability in a previously unimpaired person, beyond what might be expected from normal aging...

. It typically becomes noticeable in middle age. HD is the most common genetic cause of abnormal involuntary writhing movements called chorea. It is much more common in people of Western European descent than in those of Asian or African ancestry.