Back in his office, Dr. Mitalipov waved off those warnings. Mitochondrial DNA comprises just 37 genes, which direct the production of enzymes and molecules that the cell needs for energy, he noted. They have nothing to do with traits like eye and hair color, which are encoded in the nucleus.

“There are always people trying to stir things up,” said Dr. Mitalipov, an American citizen who grew up in what is now Kazakhstan. “Many of them made their careers by criticizing me.”

The United States is not the only country weighing mitochondrial replacement. In Britain, the government has issued draft regulations that would govern clinical trials in people. If accepted into law by Parliament, such trials, which are how banned, would be allowed to go forward, although regulators would have to license any clinical application.

Dr. Mitalipov’s fixation on mitochondria began in graduate school in Russia in the 1990s. After graduating from an agricultural institute — and a brief, unhappy stint as a manager on a collective farm — he began work on his doctoral thesis at the Research Center of Medical Genetics, a prestigious state-funded institution in Moscow. He focused on embryonic stem cells, which can be grown in the laboratory and turned into any type of cell in the body.

He noticed a strange thing. When stem cells were extracted from a mouse embryo and put in a petri dish, they stopped aging but remained healthy and growing, as if frozen in time. Somewhere in the cell, it seemed, was a clock that determined its life span.

The search for the clock took him to Utah State University for postdoctoral research in the mid-1990s. He developed an interest in cloning, a process in which the cellular clock is not only stopped but reset. Why, he wondered, do cloned animals have normal life spans?