It turns out that hereditary ataxia is not one but a group of diseases that present a similar clinical picture. So far we have identified eleven distinct spinocerebellar ataxias, and have located the precise gene responsible for the disease in eight of these disorders. For SCA1, the disease my own research focuses on, a normal CAG tract might have up to 35 repeats, while expanded tracts might encompass from forty to over eighty CAG triplet repeats. Interestingly, the longer the expansion, the more severe the disease, and the earlier its onset. The triplet repeat is unstable, so it can further expand as it is passed from generation to generation.
Expansions are more likely to occur when an affected father is passing the mutation.
This explains why fathers are more likely than mothers to generate a phenomenon known as anticipation, in which a child develops the disease at a much earlier age than the parent. While a parent may have an expanded tract of 45 CAG repeats, the child might have a longer expansion of 66 repeats, and a grandchild might have 100. Because a number of neurodegenerative disorders, including Huntington's disease, share this expansion of the polyglutamine tract, they are known collectively as polyglutamine or triplet repeat diseases. Knowing the genetic root of these ataxias allows us not only to diagnose them accurately through genetic testing, but it offers patients and their children an opportunity to plan for their future. Most importantly, it allows researchers to study these diseases at their most fundamental level.