CHAPPAQUA, N.Y.

IN the beginning, Brian and Jaclyn Rattner were focused on what their oldest child, Jarrett, could not do.

He could not talk, could not walk, did not play with toys, did not take to toilet training, missed most developmental milestones. Hoping for a diagnosis and cure, they took him to research centers at Harvard and Yale. Thinking it might be some rare genetic disorder like Tay-Sachs, which disproportionately affects Ashkenazi Jews, they had his blood shipped to Israel for analysis, and flew to England to meet with more specialists.

Ten years ago, at age 3, he was given a diagnosis of Angelman syndrome, which is characterized by mental delays, jerky body motions and a good deal of smiling and laughing  Jarrett has a sweet disposition. “But then they undiagnosed him,” Mr. Rattner said. “They didn’t know what it was, but it wasn’t that.”

And so the Rattners began focusing on who Jarrett was and what he could do. When he wanted a ball, he would pound his chest until he got it. “Sometimes, he wants to communicate so badly, you can hear him from the other room pounding his chest,” Mr. Rattner said. “There’s a lot of emotion there.”