The wind picks up dust from the unpaved road one afternoon in December as Jack van Honk turns into a ramshackle neighborhood in Lambert's Bay, on the west coast of South Africa. A stocky woman in a red patterned sundress steps out of a small home painted palest sea green, her ochre-dirt yard crowded with potted plants, many medicinal. She smiles broadly, deep wrinkles creasing a face that is cherubic and yet careworn beyond her 47 years. "Doctor! I missed you," she beams, her husky voice barely more than a hoarse whisper. Maria carries a rare genetic mutation that is almost unknown outside of southern Africa. Its effects have been to calcify a part of the brain called the basolateral amygdala , and to thicken and scar the vocal cords. A friend of Maria with the same condition lives several hours inland, and sometimes they meet when van Honk brings them to Cape Town for brain scans and other tests. "It helps to know I'm not alone," Maria says.…