Genetic analysis of Dolly's DNA has revealed she is an amalgam of two animals and is therefore a "chimera" with two genetic "mothers" rather than a pure clone.
The explanation lies in the way Dolly was created, by transferring the nucleus of an udder cell from one sheep and inserting it into another ewe's egg cell that had its own nucleus removed. Scientists have found that although all of Dolly's genes within the nucleus of her cells comes from the ewe who supplied the udder cell, the second ewe has contributed the genes of the cell cytoplasm, the part of the cell outside the nucleus.
About 0.02 per cent of a mammal's genes - including humans - exist outside the nucleus in cellular structures called mitochondria, the power-producing parts of the cell which have their own genes.
Analysis of Dolly's mitochondria by Eric Schon, of Columbia University College of Physicians and Surgeons, and Ian Wilmut, of the Roslin Institute near Edinburgh, has confirmed Dolly has two genetic mothers.
The scientists found all of the mitochondria from Dolly and nine other cloned sheep produced by the nuclear-transfer method are derived from the egg cells that received the donated nuclei. In Nature Genetics they say the results are "surprising" because some of the udder cell's cytoplasm and mitochrondrial genes must have been transferred into the enucleated egg cell.