Huntington's disease, or Huntington's chorea, is a degenerative brain disorder caused by a dominant allele. This means that if one parent has the allele, each child has a 50% chance of inheriting it. Unlike most dominant diseases, Huntington's disease is not excluded by natural selection because symptoms usually do not appear until mid-life, after the carrier has already had children. The disease is named after Dr. George Huntington, who first recognized the disease and its hereditary nature in 1872. It has been traced in numerous families in Brazil, England, and America. Genetic testing is available; however, many possible carriers opt to not test so they will not have to live with knowing that they will get Huntington's Disease later in their lives.
The abnormal allele produces a protein called huntingtin, which progressively damages cells in the basal ganglia and cerebral cortex. This damage causes involuntary movement of the face and limbs (chorea), mood swings, and forgetfulness, which get worse as the disease progresses. It is generally fatal within 20 years.