An incomplete puzzle featuring an image of a double helix, with several pieces scattered around the puzzle.

Now that science is a big step closer to being able to fiddle with the genes of a human embryo, is it time to panic? Could embryo editing spiral out of control, allowing parents to custom-order a baby with Lin-Manuel Miranda’s imagination or Usain Bolt’s speed?

News that an international team of scientists in Oregon had successfully modified the DNA of human embryos has renewed apprehensions that babies will one day be “designed.” But there are good reasons to think that these fears are closer to science fiction than they are to science.

Here is what the researchers did: repair a single gene mutation on a single gene, a defect known to cause — by its lonesome — a serious, sometimes fatal, heart disease.

Here is what science is highly unlikely to be able to do: genetically predestine a child’s Ivy League acceptance letter, front-load a kid with Stephen Colbert’s one-liners, or bake Beyonce’s vocal range into a baby.

That’s because none of those talents arise from a single gene mutation, or even from an easily identifiable number of genes....