She Laughs Too Much No, not the smiling sister, but the 10-year-old on the right, who has Angelman syndrome. She inherited it from her mother’s chromosome 15. Fortunately, her two siblings inherited the mother’s other chromosome 15 and are normal. If she had inherited the identical deletion on her father’s chromosome 15, she would have developed Prader-Willi syndrome, which would cause her to be overweight as well as always hungry and often angry. With Angelman syndrome, however, laughing, even at someone’s pain, is a symptom.