Geneticists reported Wednesday that they had crossed a threshold long considered off-limits: They have made changes in human DNA that can be passed down from one generation to the next.The researchers at the Oregon Health & Science University in Portland say they took the step to try to prevent women from giving birth to babies with genetic diseases. But the research is raising a host of ethical, social and moral questions."That kind of genetic engineering has been ruled off limits," says Marcy Darnovsky of the Center for Genetics and Society. "And it's a very bright line that has been observed by scientists around the world."There's been lots of reasons for that line. One big one is purely practical, says Dartmouth bioethicist Ronald Green "If we make mistakes, we'll effectively be introducing a new genetic disease into the human population — for generation after generation," Green says.But beyond the risks, Green says taking that step has long raised more far-reaching fears. It's the kind of technology that could be used to try to create genetically superior...