To Build A Baby

The extraordinary thing about Molly Nash is that she seems like a typical second grader in Englewood, Colorado. "She can be as stubborn as an ox," says Lisa Nash, her mother, "and she smarts off now and then." But like most 8-year-olds, she has redeeming qualities--a round, cheeky face, a toothy smile, brown bangs. She also takes dance lessons and plays soccer, and she's a whiz in reading and math. "She's a bit small for her age," says Nash. "But not extremely small. There are kids in her class who are smaller."

Smallness is a vestige of Molly's tentative start in life. For a while Molly grew far too slowly, and the odds were good that she wouldn't live much beyond the age of 6. She had been born with a rare disorder called Fanconi's anemia, which was causing cells in her bone marrow--the ones that produce white blood cells and other defenses against infection--to fail. Molly needed new ones from a donor who was an almost exact genetic match. That meant that her parents needed to have another child, and fast. The trouble was, the odds of having one with the right genes and who didn't himself have the disease were only one in four.

Fortunately for Molly, there was a way of loading the genetic dice. Mark Hughes, a molecular biologist, has worked for the past 10 years building and perfecting the genetic equivalent of a one-hour photo-developing shop. If parents want a child with certain genes, doctors first use techniques of in vitro fertilization to make dozens of embryos--however many it takes to ensure that one of them has the desired genes. Hughes's technique, called preimplantation genetic diagnosis, then tells you which embryo to pick. What's more, Hughes can perform the test in 24 hours, with time to spare for implanting the embryo into the womb. The Nashes used PGD to conceive Adam, now 2-1/2, who successfully donated umbilical-cord blood to save Molly's life.

Molly wasn't the first child to benefit indirectly from PGD, and she won't be the last. While the world panics over false claims of human cloning, PGD is quietly transforming reproductive medicine by giving parents unprecedented control over what genes their offspring will have. So far the technique has been used largely, as in Molly's case, in laudable efforts to avoid passing along single-gene inherited diseases. But PGD makes some people nervous, because it also gives doctors a rudimentary ability to manipulate traits--the morally reprehensible practice of eugenics. At present, manipulating complex traits like intelligence or athletic ability is impossible, but it may not always be. The fear is that as other aspects of reproductive technology improve, PGD may be misused. "There are 900 genetic tests available or in development," says Kathy Hudson, director of the Genetics and Public Policy Center at Johns Hopkins University in Baltimore, and a fellow of the World Economic Forum. "Determining which of these tests to offer to whom and at what point is really complex."

Are the benefits worth the risk? Hughes, Molly's parents and many others think so. PGD has in recent years moved into the mainstream of reproductive science. Clinics in London, Chicago, Tel Aviv and Brussels have recently begun to offer the procedure. Although Hughes doesn't keep count, his personal computer lists dozens of obstetricians who've sent patients to his lab.

The process starts with the arrival of tiny plastic tubes packed in ice, each containing a single human stem cell plucked a few hours before from a three-day-old embryo. The cells come from fertility clinics, where would-be parents have their eggs harvested, fertilized and grown in petri dishes. By day three a human egg cell has managed to divide, on average, into only six stem cells. To find out if it carries the genes for Tay-Sachs or cystic fibrosis or sickle-cell anemia, the lab's 60 researchers and technicians copy the sample cell's DNA and analyze it with a Willy Wonka assortment of specialized machines. The trick is in coming up with clever ways of finding specific genes quickly, starting with only a single sample of DNA. Behind panes of glass, robotic hands shuffle trays of a hundred tiny test tubes.