Huntington's disease

Huntington's is a neurodegenerative disease that most recognizably causes memory issues and uncontrollable movements in the limbs. After symptom onset (somewhere between age 30 and 50), those affected by Huntington's typically live another 15 to 18 years as their symptoms worsen. There is no cure. Huntington's is passed genetically from parent to child. The gene that causes Huntington's works in such a way that if a parent has the disease there is a 50/50 chance that their child will get it. Huntington's cannot be passed on recessively, meaning it cannot skip a generation. In other words, if someone has a mother or father with Huntington's, but they themselves did not receive it from that parent, their children (and their grandchildren, and so on) will also never have it. Until the early 1990s it was impossible to test for Huntington's. Instead, symptoms would usually appear later in life. This would often be after the person affected already had children. It would then be impossible to know if their children had it, as it was a 50/50 chance. In 1993, due to determining which gene causes Huntington's, it did become possible to test children for it and know if the disease was passed on or not. For a variety of complex reasons, however, children are considered unable to consent to such a test and are therefore very rarely tested unless they're showing early symptoms.