The birth of Dolly the sheep 20 years ago proved that DNA from an adult cell of a mammal could be transferred to an unfertilized egg and give rise to an animal genetically identical to the donor. Yet Dolly died prematurely, which left the impression that cloned animals have shorter life spans.

To figure out if clones are inherently less healthy than their “natural” counterparts, University of Nottingham developmental biologist Kevin Sinclair followed four of Dolly's clones—Debbie, Denise, Dianna and Daisy (above)—from birth through middle age. All four were derived from the same batch of frozen mammary gland cells as Dolly was. Sinclair and his colleagues also monitored nine cloned sheep of other breeds. The 13 ruminants are now more than nine years old—the equivalent of a human's seventh or eighth decade—and all are as healthy as regular sheep, according to scans of their bones, blood glucose readings and detailed blood pressure monitoring. “We can say these [clones] are perfectly normal,” Sinclair says. The results were published in Nature Communications.

So why did Dolly die young? The scientists who worked with her say she died from a contagious illness that spread through the flock—not because of any problem specific to clones. She did have some arthritis in her knees, but geneticist Helen Sang of the Roslin Institute in Edinburgh, where Dolly was born, notes that any sheep kept inside and fed treats as often as Dolly was would be subject to joint problems.

Two decades later cloning is still less efficient than normal breeding. But the new study shows that if a cloned animal survives gestation and is in good health during the first few weeks of life, it is likely to have the same chances of thriving as other animals of its breed. Cloning is used today to generate embryonic stem cells for research and to breed high-value livestock. Long live the copies!