A pressing question, said Rudolf Jaenisch, an M.I.T. biology professor, is why anyone would want to edit the genes of human embryos to prevent disease. Even in the most severe cases, involving diseases like Huntington’s in which a single copy of a mutated gene inherited from either parent is enough to cause the disease with 100 percent certainty, editing poses ethical problems. Because of the way genes are distributed in embryos, when one parent has the gene, only half of the parent’s embryos will inherit it. With gene editing, the cutting and pasting has to start immediately, in a fertilized egg, before it is possible to know if an embryo has the Huntington’s gene. That means half the embryos that were edited would have been normal — their DNA would have been forever altered for no reason. “It is unacceptable to mutate normal embryos,” Dr. Jaenisch said. “For me, that means there is no application.”

Noting the many unresolved questions about gene editing of human embryos, a group of leading American researchers recently published a paper in the journal Science calling for a moratorium on doing such work for clinical purposes. They pointed out that current knowledge about genes and their interactions was limited and that changing a disease gene in an embryo that then develops into a baby could have unintended consequences that would be inherited by all of that person’s progeny.

A recent paper in the journal Nature made similar points. In it, Edward Lanphier of Sangamo Biosciences in Richmond, Calif., and his colleagues wrote: “In our view, genome editing in human embryos using current technologies could have unpredictable effects on future generations. This makes it dangerous and ethically unacceptable.”

In an interview, Mr. Lanphier added, “It literally boils down to, How do you feel about the human race and the human species?”