Twenty years after Dolly the sheep was born out of a test tube, four sheep cloned from her DNA are healthy and aging normally, according to a study published this week. The findings, published in the journal Nature Communications, may dispel long-held concerns that cloned animals age prematurely.

Many of those concerns arose after Dolly was cloned. The sheep appeared to age faster than normal, and suffered from osteoarthritis in her knees and hips at an early age. After she was diagnosed with an incurable lung virus, veterinarians decided to put her down, at the age of six. Widespread concerns over the health effects and ethics of cloning led to a UN ban on human cloning in 2005; several countries have also banned reproductive cloning of animals.

"They're old ladies. They're very healthy for their age."

But the study published this week suggests that other cloned animals may not suffer the same problems that Dolly did. As The New York Times reports, researchers from the University of Nottingham conducted a range of health tests on 13 cloned sheep — including four derived from the same DNA strand as Dolly — and found that they're mostly healthy. Their blood pressure, heart function, metabolism, and joints did not show signs of premature aging, and although some suffered from slight arthritis, the authors say it's no cause for concern.

"They’re old ladies," Kevin Sinclair, a developmental biologist and lead author of the study, tells the Times, referring to the four Dolly clones. "They’re very healthy for their age."

It's unclear whether the study's findings will have any impact on how cloning is perceived. There are still health risks associated with the procedure, including birth defects and miscarriages, and the debate over ethics will certainly continue. But Sinclair and his colleagues say their study — the most thorough to date — should at least ease concerns over premature aging. The sheep tested in their experiment will be put down when they reach the age of ten, at which point the researchers will conduct a closer post-mortem study.