 She has her mother's laugh. The powers, perversions, and potential of heredity, by Carl Zimmer, read for you by Joe Ackman. To Grace, for spending this juncture between the past and the future with me. The whole subject of inheritance is wonderful, Charles Darwin. Prologue. The worst scares of my life have usually come in unfamiliar places. I still panic a bit when I remember traveling into a Sumatran jungle only to discover my brother Ben had dengue fever. I lose a bit of breath any time I think about a night in Bujumbura where a friend and I got mugged. My fingers still curl when I recall a fossil-mad paleontologist leading me to the slick, mossy edge of a Newfoundland cliff in search of pre-Cambrian life. But the greatest scare of all, the one that made the world suddenly unfamiliar, swept over me while I was sitting with my wife, Grace, in the comfort of an obstetrician's office. Grace was pregnant with our first child, and our obstetrician had insisted we meet with a genetics counselor. We didn't see the point. We felt untroubled and being carried along into the future, wherever we might end up. We knew Grace had a second heartbeat insider, a healthy one, and that seemed enough to know. We didn't even want to find out if the baby was a girl or a boy. We would just debate names in two columns, Liam or Henry, Charlotte or Catherine. Still, our doctor insisted, and so one afternoon we went to an office in Lower Manhattan where we sat down with a middle-aged woman, perhaps a decade older than us. She was cheerful and clear, talking about our child's health beyond what the thromb of a heartbeat could tell us. We were politely cool, wanting to end this appointment as soon as possible. We had already talked about the risks we faced starting a family in our thirties, the climbing odds that our children might have Down's syndrome. We agreed that we'd deal with whatever challenges our child faced. I felt proud of my commitment. But now, when I look back at my younger self, I'm not so impressed. I didn't know anything at the time about what it's actually like raising a child with Down's syndrome. A few years later, I would get to know some parents who were doing just that. Through them, I would get a glimpse of that life, of round after round of heart surgeries, of the struggle to teach children how to behave with outsiders, of the worries about a child's future after one's own death. But as we sat that day with our genetics counselor, I was still blithe, still confident. The counselor could tell we didn't want to be there, but she managed to keep the conversation alive. Down's syndrome was not the only thing expectant parents should think about, she said. It was possible that the two of us carried genetic variations that we could pass down to our child, causing other disorders. All complete. Ready to continue?