Where Is Human Evolution Heading?

The race's DNA is changing faster than ever; what it means for our descendants.

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It didn't take long for prospective parents to realize that the same method could be use to sort embryos for other reasons. Since 2000, parents have been able to use PGD to choose an embryo's tissue type, so that the ensuing child could serve as a stem cell or bone marrow donor to a sick sibling. More recently, a few have used PGD to reject embryos that have genes that merely increase the risk of disease in adulthood, such as the BRCA breast cancer genes. A few parents with disabilities such as deafness have used PGD to choose a deaf child. And PGD is increasingly used to reject embryos that have no problems at all—unless you consider being the wrong sex a problem. A number of fertility clinics in the United States advertise PGD to parents who want to be guaranteed the child will have the sex they choose. One California clinic boasts of "over 3,800 cases: 100 percent sex selection success." With PGD largely unregulated in the United States, it doesn't take a Nobel Prize in genetics to imagine that babies could soon be ordered up in custom sizes and colors, like a Mini Cooper.

The next step: children with genes from three parents. In the late 1990s, IVF clinics started injecting cytoplasm from younger women's eggs into those of older women, in an effort to increase the odds of pregnancy. About 30 babies have been born worldwide as a result, and those children carry genes from both women. But that rejiggering of the human germline was almost inadvertent. Scientists are now intentionally making that mix. Earlier this year, researchers at Newcastle University in England deliberately created human embryos that had DNA from one father and two mothers, in order to avoid the risk of a mitochondrial disease from the original mother.

But it's too early to lie awake worrying that genetically manipulated superkids are going to ace your grandkids out of varsity soccer, says Thomas Murray, a bioethicist and president of the Hastings Center. "Our capacity to do these kinds of intentional designs is vastly overrated." But, he says, it's not too early to start thinking about what's really important about being a parent. The traits that people most value, Murray says—being smart, being kind, being a successful competitor—are the ones least likely to be determined by a few tweakable genes. For that kind of control over the next generation, it still takes good old-fashioned nurturing, teaching, and love.