CAMBRIDGE, Mass. — When Sharif Tabebordbar was born in 1986, his father, Jafar, was 32 and already had symptoms of a muscle wasting disease. The mysterious illness would come to define Sharif’s life. Jafar Tabebordbar could walk when he was in his 30s but stumbled and often lost his balance. Then he lost his ability to drive. When he was 50, he could use his hands. Now he has to support one hand with another. No one could answer the question plaguing Sharif and his younger brother, Shayan: What was this disease? And would they develop it the way their...