Linde Jacobs paced back and forth across her bedroom, eyeing the open laptop on the dresser and willing the doctor to appear. Her husband was dropping off their older daughter at school. Their younger daughter was downstairs, occupied by a screen. Linde wanted to be alone when she learnt whether she carried the family curse.

Linde’s mother, Allison, had died just four weeks before, after a mutant gene gradually laid waste to her brain. In her 50s, Allison transformed from a joyful family ringleader into an impulsive, deceptive pariah. She drove like a maniac on cul-de-sacs. She pinched strangers, shoplifted craft supplies and stole money from her daughter.