The Truth About Dolly the Cloned Sheep

A study of the famous animal’s bones suggests the conventional wisdom about how clones age is probably wrong.

Dolly surrounded by a flock of photographers
Dolly the sheep meets the press in February, 1997. (Jeff J Mitchell / Reuters )

Dolly the sheep was the first animal to be cloned from an adult cell, and like many firsts, she came to stand in for all of her kind.

So when scientists suspected she had short telomeres—stretches of DNA that normally shorten with age—people wondered if it was because she was cloned from an adult cell. When she started to limp at age five, headlines said that her arthritis “dents faith in cloning.” And when she died at age six—as the result of a common lung virus that also killed other sheep in her barn—her short life again became a parable about cloning. A certain narrative took hold.

Then last year, Kevin Sinclair, a developmental biologist at the University of Nottingham, published a paper about several clones including Dolly’s four “sisters,” who were created from the same cell line as Dolly and lived to the old age of eight (about 70 in human years). They were quite healthy for their age. So of course, he kept getting questions, like if these animals are so healthy, then why was Dolly so unhealthy? It was Dolly that everyone cared about.

Sinclair would point out that Dolly was not so unhealthy. But the questions inspired his team to go looking for Dolly’s health records from the early 2000s. The records, however, were lost. “Everything has moved on. People had moved away, and people are doing other things,” says Sinclair. But after her death in 2003, Dolly’s bones were turned over to the National Museum of Scotland. Sinclair’s team got permission to study them—along with the bones of Megan and Morag, two sheep cloned from non-adult cells who were prototypes for Dolly, and Dolly’s naturally conceived daughter Bonnie.

A team of veterinarians scored X-rays of the bones for signs of arthritis. Megan and Bonnie, who had died at the ripe old ages of 13 and nine, respectively, did indeed have signs of arthritis, which was normal for their age. Megan, who had died at age four in an earlier outbreak of same lung virus that killed Dolly, did not. Even Dolly’s knee did not show signs of arthritis.

Setting up an X-ray of a cloned sheep’s bones (University of Nottingham)

Arthritis also affects the soft tissue in a joint, so Sinclair notes the bones alone do not point to a conclusive diagnosis or lack thereof. (And Dolly really did limp.) But the overall set of data from Megan, Morag, and Bonnie as well as Dolly’s elderly sister clones suggest arthritis is no more common among clones than ordinary sheep. Fears about prematurely aging clones may be greatly exaggerated. “We felt the record needed to be set straight,” says Sinclair.

Even the fears about Dolly’s too-short telomeres haven’t quite borne out. Telomeres are repetitive DNA sequences at the ends of chromosomes, and they are shortened every time a cell divides. In 1999, scientists published data suggesting Dolly’s telomeres were too short for her age. Since then, scientists have cloned a whole menagerie of animals: mice, horses, cattle, pigs, dogs, and so on. Studies of their telomere lengths have turned up every possible result: Clones have shorter telomeres, clones have longer telomeres, and clones have normal telomeres—depending on the species or cloning technique.

Clones do have unique health problems, just not the ones that dominated headlines about Dolly. Clones are less likely to make it to term in pregnancy, and when they are born, they are more likely to be a little maladjusted. “You have to baby them—give them oxygen, give them glucose until they normalize,” says George Seidel, who studies animal-reproduction technologies at Colorado State University. The clones that make it to adulthood are generally pretty normal. But that makes it extremely expensive to clone livestock, on the order of $20,000 each. Despite much ado about cloned meat, cloning is not an economical way to produce cattle for meat or milk.

It does have niche applications in the livestock industry, though, like recreating million-dollar bulls with extremely valuable genes. Seidel, who also has an cattle ranch, buys semen from Final Answer 2, the clone of the famed sire Final Answer who died in 2014. Semen from the right bull is also incredibly valuable to the dairy industry. “You’ve probably had some cheese from the offspring of a clone,” says Seidel. But actual clones are not sold for food in the United States.

For all the attention Dolly once attracted, cloning itself has not radically altered the world. The research that created her did teach scientists a lot about how embryos can be manipulated, which has inspired new lines of research into stem cells.

Dolly’s four sister clones—Daisy, Debbie, Denise, and Dianna—were actually euthanized last year. Sinclair’s group is still studying their preserved cells and tissues for answers to question like the unresolved debate over telomere length. But it’s the end of a certain era. “You get to know them very well,” says Sinclair about Daisy, Debbie, Denise, and Dianna. “Even though they’re clones they have their own personalities, and this is an important point to make.” They may have started with the same DNA as Dolly, but they were not the same as her.