Jeff Carroll had been married for six months when he and his wife decided not to have children. Carroll, 25 years old and a former corporal in the US Army, had just found out that he had the mutation that causes Huntington’s disease, a genetic disorder that ravages the brain and nervous system and invariably ends in an early death. He had learnt that his mother had the disease about four years earlier, and now he knew that he was all but certain to develop it, too.

Faced with a 50% chance of passing on the same grim fate to their children, the couple decided that kids were out of the question. “We just kind of shut that down,” says Carroll.

But he had begun studying biology in the army in the hope of learning more about the disease. He found out about a process called preimplantation genetic diagnosis or PGD. By conceiving through in vitro fertilization (IVF) and screening the embryos, Carroll and his wife could all but eliminate the chance of passing on the mutation. They decided to give it a shot, and had twins free of the Huntington’s mutation in 2006.

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