U.S. medical advisers are weighing whether there is scientific justification for allowing human studies of a controversial procedure known as "three-parent in vitro fertilization (IVF)," a technique supporters say could prevent horrific genetic defects, but critics believe could lead to designer babies.

Over two days of public hearings starting on Tuesday, scientists were scheduled to present their research to outside advisors to the U.S. Food and Drug Administration. The agency will decide whether safety concerns raised by three-parent IVF are minimal enough to allow clinical trials to begin.

The committee is focusing only on the science, not the legal or ethical issues, Celia Witten of the FDA's Office of Cellular, Tissue and Gene Therapies said in comments introducing the hearing.

That did not stop members of the public from focusing on ethical issues. Speakers warned the panel that use of three-parent IVF "could alter the human species," represented "an unprecedented level of experimentation on non-consenting human subjects" (meaning any children born via the technique), and "could open the door to genetically-modified children" who would be "manufactured products."

In the three-parent procedure, one man would donate sperm and all its DNA for in vitro fertilization. The would-be biological mother would contribute the egg and most of its DNA. But if the mother carries harmful genetic mutations in cellular structures called mitochondria, scientists would remove her unhealthy mitochondria and substitute those of a second woman so the baby would not inherit a potentially devastating "mitochondrial disease."

Allowing such procedures "would produce genetically modified human beings," Marcy Darnovsky, executive director of the Berkeley, California-based Center for Genetics and Society, a non-profit that focuses on genetic and reproductive technologies, told the committee.

If the FDA allows clinical trials, she warned, it would introduce "a regime of high-tech consumer eugenics" and represent "the first time a government body had okayed genetic changes for humans and their descendants."

Although the FDA committee is considering only scientific issues, such as whether animal research can show whether mitochondrial manipulation is safe, the agency said it is prepared to go beyond that.

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