WASHINGTON — Ever wonder why sharks get several rows of teeth and people only get one? Some geneticists did, and their discovery could spur work to help adults one day grow new teeth when their own wear out.

A single gene appears to be in charge, preventing additional tooth formation in species destined for a limited set. When the scientists bred mice that lacked that gene, the rodents developed extra teeth next to their first molars — backups like sharks and other non-mammals grow, University of Rochester scientists reported Thursday.

If wondering about shark teeth seems rather wonky, consider: Tooth loss from gum disease is a major problem, here and abroad, and dentures or dental implants are far from perfect treatments. If scientists knew exactly what triggers a new tooth to grow in the first place, it's possible they could switch that early-in-life process on again during adulthood to regenerate teeth.

"It's exciting. We've got a clue what to do," said Dr. Songtao Shi of the University of Southern California School of Dentistry, who said the Rochester discovery will help his own research into how to grow a new tooth from scratch.

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