Machado-Joseph disease is found mainly among people of Portuguese descent. Hemochromatosis causes a build-up of iron in the heart, liver, and kidneys, eventually leading to death in middle age through iron overload. It affects mostly males, becuase women dispose of the excess iron through menstruation. Huntington disease is a fatal neurodegenerative disease that usually appears around age 40 and leads to death in 10 to 20 years. It is autosomal dominant, which means that people who develop it have one parent who has the disease.
Myotonic dystrophy causes a failure of the muscles' ability to release after use.
Often this is the only symptom which may appear in late middle age. In severe cases, important muscles, including the heart, may be affected. Myotonic dystrophy is thought to account for some fatal heart attacks while sleeping. The size of the expansion may predict the severity or age of onset. Sex determination analysis is not sex selection.