More people are now looking up their family trees, eager to learn more about their ancestors. But, in a surprising twist, scientists are using family trees to track disease-causing genes.

They are going back in history, sometimes hundreds of years, to track the person who first produced the faulty genes. They're then tracking down other lines of the family tree, to find the modern-day descendants of this person and to warn them of the health dangers they face.

This genetic genealogy, as the science is known, is being developed as a new approach to preventing - and treating - conditions with a strong genetic link such as breast cancer, osteoporosis and depression, as well as rare syndromes that cause fatal insomnia and an inability to feel pain.

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