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Dolly, the first animal to be cloned from an adult of its species, was born 20 years ago today at the Roslin Institute in Scotland. When her creators announced what they had done, it triggered warnings of rich people cloning themselves for spare parts, of tyrants cloning soldiers for armies, of bereaved parents cloning their dead child to produce a replacement — and promises that the technique would bring medical breakthroughs. Which raises some questions:

Why are there no human clones?

Because of scientific, ethical, and commercial reasons.

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The scientists who created Dolly — named after Dolly Parton, naturally — removed the DNA from a sheep ovum, fused the ovum with a mammary epithelial cell from an adult “donor” sheep, and transplanted the result, now carrying DNA only from the donor, into a surrogate ewe. But that technique, called somatic cell nuclear transfer (SCNT), turned out not to be so easy in other species.

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