John Zhang, shown in the lab at New Hope Fertility Center in New York. Washington Post photo by Carolyn Van Houten

Washington - This is John Zhang, the Chinese-born, British-educated founder and medical director of a Manhattan fertility centre that is blowing up the way humans reproduce.

In 2009, Zhang helped a 49-year-old patient become the world's oldest known woman to carry her own child. In the not-too-distant future, he says, 60-year-old women will be able to do the same.

In 2015, Zhang stunned his scientific peers by transferring a genetically "abnormal" embryo to the womb of a woman who had run out of other options. Abnormal embryos - which appear to have the wrong number of chromosomes - are almost universally considered nonviable and discarded by other fertility doctors. The woman gave birth to a healthy baby girl, prompting clinics around the world to reevaluate their policies.

But it was the three-parent baby that really put Zhang on the map. Working with a Jordanian couple who had lost six babies - two in infancy, four in miscarriages - to Leigh syndrome, a heritable neurological disorder, Zhang put to practical use a procedure that others had dared to try only on animals.

He extracted the woman's nuclear DNA, which carries the biological material responsible for such things as physical appearance and other major traits, but not the ones that lead to Leigh disease. Then he inserted the DNA into a healthy donor egg and fertilised it with sperm from the woman's husband. The child, a boy born in 2016, appears to be free from the disease.

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"If there is a gene which causes a problem, it would be washed out through natural evolution. Eventually, these kind of babies are not going to be born. That is how nature selects," Zhang said in a recent interview. "But if we can alter the gene, why can't we alter it?"

For some, the "three-parent baby" was a joyous miracle of 21st-century medicine. In a feature about 10 people who matter in science, the journal Nature dubbed Zhang a "fertility rebel." For others - including US regulators - the baby's birth marked an unnerving step down the slippery slope of tinkering with human life in ways that are not fully understood.

In the countless news reports and opinion pieces that followed, writers made comparisons to the dystopian worlds of Gattaca - a movie in which babies conceived the natural way are believed to be inferior to those who are genetically manipulated in a lab for intellect and athletic ability - and Orphan Black - a TV show that raises the specter of genetically designed clones.

Fertility specialist John Zhang famously helped a couple conceive a baby with the DNA of three people in order to prevent the child from being born with a genetic disease. Courtesy of New Hope Fertility Center

It didn't help that Zhang's company soon began to market the technology through his two companies, New Hope Fertility Center and Darwin Life, a biotech start-up. They offered to take DNA from older women and put it into donor eggs from younger women so that women of almost any age could bear their genetic children. The idea caused a sensation at a time when many women are delaying childbirth and then having trouble bearing children after the age of 40.

Zhang predicted that the ability to edit genes and transfer DNA will transform the world as quickly and dramatically as the tech revolution, that shift from computers that occupied entire rooms to smartphones that fit in everyone's hip pocket. He noted that, initially, one smartphone GPS service erroneously depicted the Brooklyn Bridge with a sharp, roller-coaster-like plunge.

"In the beginning, if mass technology misleads us, it is totally normal," he said. "That doesn't mean this technology is useless. You just have to know how to use it."

When asked about technological glitches in the making of human babies - when a mistake could mean the difference between life and death or a good life and one of suffering - he became thoughtful.

"You have to discuss with the patient what are the risks, what is the worst things that could happen." He looked sad as he described the darkest scenarios. "The worst thing that could happen is to have a baby that is abnormal. The least worst is to have a miscarriage."

Still, he said he believes doctors should "not be gatekeepers, but educators and counsellors."