What they found surprised them and may provide useful information to researchers who study inherited diseases like neuromuscular and kidney problems, which are passed down on the mother's side only.

Scientists are sure the DNA in the nuclei of Dolly's cells is the same as the genetic material in the adult sheep from which she was cloned.

What was not certain was whether the so-called mitochondrial DNA in Dolly's cells was also copied from that animal.

Dr Eric Schon: "Surprised"

Dr Eric Schon from Columbia University has now shown that it is not.

The mitochondria are very special and hugely important features in cells.

They are often called "powerhouses" because of their role in providing the energy that drives the biochemical processes in the body.

They stand apart from other sections of the cell because they have their own unique genome - their own packet of DNA - that is separate from that of the nucleus.

When Dolly was created by a method called nuclear transfer, the nucleus of a donor cell was fused with a recipient egg cell that had its nucleus removed.

World famous

During this process, the cytoplasms - the cellular "soup" in which mitochondria reside - of the two different cells were merged.

Theoretically, therefore, Dolly's mitochondria could have derived from the donor cell, the recipient cell or both.

But when Dr Schon looked at the cells of the world famous sheep and nine other clones, he found that, in all cases, the mitochondrial DNA comes exclusively from the recipient egg cell.

Greater clarity

One of the key and little understood features of mitochondrial DNA in normal animals is why it is passed exclusively from mother to child with no mixing of similar genetic material from the father.

When this genetic material is defective, it can lead to the mother passing on neuromuscular and kidney diseases to her young.

Dr Schon believes his work may offer another route to investigate these problems.

He told the BBC: "If we understood how that mechanism operated that would allow us to manipulate cells and study mitochondrial genetics with much greater clarity, this might allow us to develop new treatment strategies based on our ability to manipulate the mitchondria in cells."

The research, which was conducted with the aid of Dolly's creators in Scotland, is published in Nature Genetics science journal.