That is because the present method of cloning does not eliminate all genetic material from the donor egg - nuclear DNA in the donor egg is replaced entirely by the DNA from the animal we wish to clone, but the mitochondrial DNA (floating in the cell and not in the nucleus) will still be that of the donor.
That is why identical twins look exactly the same while clones do not. Unless, of course, you clone a female with her own egg.
However, using nuclear transfer technology, the recipient individual's nuclear DNA is now derived asexually from another, single embryo.
If this new fertilized egg is allowed to begin dividing and growing, and if this cluster of growing egg cells is then separated into individual egg cells again, and each of these allowed to divide and grow into separate individuals, then it is possible to have two or more embryos sharing identical nuclear DNA . These individuals are now called, somewhat misleadingly, "embryo clones" . (See "Genetic Cloning versus Genetic Twinning" ingly, for more discussion on the distinction between clones and twins.) Created via the asexual reproduction of nuclear transfer technology, they each share identical, cloned, nuclear DNA.
However, there is other DNA, called mitochondrial DNA , that is not found in a cell's nucleus, but in the cytoplasm of the cell that surrounds the nucleus. This mitochondrial DNA will not be transferred from the donor embryo to the recipient egg cell because only the nucleus of the embryo is transferred during nuclear transfer. Thus the donor embryo and the recipient egg cell will not have identical mitochondrial DNA.
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