Scientists experiment with "turning off" excess chromosome that causes Down's

Down's syndrome happens when a human being ends up with an extra copy of chromosome 21 — three copies, instead of the normal two. But scientists say they might have found a way to make that extra chromosome functionally irrelevant. If they're right, it could lead to treatments that could someday reduce the symptoms of Down's. The trick is connected to another extra chromosome that the human body "turns off" all the time — the X. Women have two X chromosomes, of course, but only one ever gets to express itself. Scientists put the same mechanism to use on chromosome 21 in petri dish experiments.