The discovery that Dolly the cloned sheep has arthritis has raised fears that the cloning process may cause premature ageing, calling into question the ethics of producing animals destined to suffer unnecessarily.
Professor Ian Wilmut, who led the scientists at the Roslin Institute near Edinburgh in cloning Dolly in1996, said it was unusual for a sheep under six years old to develop arthritis in the rear legs and it could be the result of her being cloned from a 6-year-old ewe.
"We know already that there's an unusual incidence of death of cloned animals around the time of birth," Professor Wilmut said.
"What we need to go on studying is whether diseases like arthritis, which tend to be associated with older age, occur in a normal way or whether the incidence is changed.
"That Dolly has arthritis at this comparatively young age suggests that there may be problems. We do not know and it's very important that we look."
Dolly became lame in her left hind leg a few weeks ago.
She was examined by veterinarians from the University of Edinburgh and x-rays confirmed she has arthritis in the hip and knee of that leg.
She is being treated with anti-inflammatory drugs.
Tim King, the university vet who examined her, said: "Arthritis in sheep is quite common, but it is comparatively unusual for it to be in these joints. The elbow is the most commonly affected joint."
Dolly made history when her birth was announced in 1997.
She is the first mammal to be cloned from the cell of an adult animal and her birth raised the prospect of humans being created in the same way.
The history of cloning research is littered with examples of the "inefficiency" of the process, which translates into failed embryos, oversized foetuses, stillbirths and congenital abnormalities.
Dolly herself was the result of 277 cloning attempts.
She was created from a cell taken from the udder of a 6-year-old ewe and has produced six healthy lambs of her own, conceived normally.
