Nancy Wexler was 22 when she got the grim news. Arriving home in Los Angeles after studying abroad on a Fulbright scholarship, she learned that her mother, then 53, had been found to have Huntington's disease. Wexler was devastated. The genetic disorder, which afflicts 30,000 Americans, had claimed the lives of her three uncles and her maternal grandfather, and she was only too well aware of what lay ahead for her mother: mental deterioration, uncontrollable movements in all parts of the body and, after a decade or so, death.
Because her mother had Huntington's, Nancy realized that she herself had a...