Genetic analysis of Dolly the sheep's cells has revealed that the genes in her mitochondria - the powerhouses of cells - have come from another sheep involved in the cloning experiment.

In the cloning technique of cell nucleus replacement (CNR), a donor cell nucleus is fused with a donor egg stripped of its own nuclear DNA. Dolly is a clone because the DNA in the nuclei of her cells are identical to that of the ewe whose udder cell was used in the experiment. But a report by researchers at Columbia University College of Physicians and Surgeons in New York and the Roslin Institute in Scotland show that all Dolly's mitochondrial DNA came from the the donor egg with which the udder cell was fused.

These results have implications for developing therapies for the rare but fatal human mitochondrial diseases. Although there are only 37 genes in human mitochondria, they control almost all energy metabolism in each cell. Dr Eric Schon, professor of genetics and development in neurology at Columbia expected to see about 2-5% of mitochondrial DNA from the udder cell in Dolly's mitochondrial DNA, but was surprised to find that there was no contribution from the udder cell. Every child born to a woman suffering from a mitochondrial disorder will suffer devastating effects that shortens their life span to 15 to 20 years.

This report could pave the way to the use of cell nucleus replacement techniques to prevent the transmission of a woman's mitochondrial DNA defect to her children.