Dolly the cloned sheep could be genetically older than her years, according to follow-up research carried out by the Roslin Institute and its commercial arm PPL Therapeutics, where Dolly-style cloning was developed. Dolly may look like a three year old ewe, but the tips of her chromosomes - called telomeres - show age-related erosion that is characteristic of older animals. However, the researchers point out that the telomere evidence is somewhat ambiguous as variation in telomere length between individual sheep may be entirely normal.

Alan Colman, Research Director of PPL, said that although it is clear that telomeres get shorter with age, it is a reflection of ageing and may not have any effect on the actual processes of ageing. Bonnie the lamb, born to Dolly last year after she mated with a conventionally bred ram, has normal telomeres - consistent with the idea that sexual reproduction relengthens telomeres for the next generation.

Once again, this research raises yet further doubts about the feasibility of human reproductive cloning - a procedure that is not only illegal in the UK but has been repeatedly dismissed by Roslin scientists as unsafe for human beings.