Humanity has long dreamed of perfection, striving to be faster, stronger and brighter, pushing nature to the limit. Four centuries before people were conceived in a petri dish, Swiss alchemist Paracelsus claimed flawless little beings could be grown in pumpkins filled with urine and horse dung, but there is no record he produced a crop.

With the birth of Louise Brown in 1978, the test tube finally succeeded where the pumpkin had failed, and the year she turned 11, scientists moved beyond making life in a lab: They found a way to peer into an embryo's genes and predict what that life might be like.

That ability is now morphing into a whole new approach to baby-making, one that gives people an unprecedented power to preview, and pick, the genetic traits of their prospective children.

Just as Paracelsus wrote that his recipe worked best if done in secret, modern science is quietly handing humanity something the quirky Renaissance scholar could only imagine: the capacity to harness our own evolution. We now have the potential to banish the genes that kill...