Reproductive cloning is a technology used to produce a genetically identical copy of another living thing. Two techniques used to do this are artificial embryo "twinning" and somatic cell nuclear transfer (SCNT). Artificial embryo twinning mimics the natural way twins develop. Scientists separate an early-stage embryo into two or more individual cells, and each cell is left to divide and develop into an embryo. The embryos are implanted in the uterus of a female surrogate and carried to term. Because the twinned embryos come from the same zygote, they are genetically identical.

In SCNT, scientists transfer genetic material from the nucleus of a somatic cell to an egg cell whose nucleus has been removed. (A somatic cell is any body cell, except for a male or female sex cell.) To stimulate cell division, either electrical current or special chemicals are applied to the modified egg. As with twinning, the cloned embryo is eventually implanted into the uterus of a surrogate female and carried to term.

Dolly, or any other animal cloned using SCNT, is not an exact replica of the donor animal. Because some of the clone's genetic materials come from the host egg cell's mitochondria, only the clone's nuclear DNA matches the donor's. Also, because the environment influences physical traits and personality, a clone's appearance and character will inevitably differ from the donor's. In addition, cloned animals like Dolly have shown signs of premature aging.

Hundreds of animal clones developed through SCNT have been born since Dolly, including other sheep, goats, cows, mice, pigs, cats, and rabbits. But the success rate for reproductive cloning is low. In fact, attempts at cloning other species—such as monkeys, chickens, and horses—have been entirely unsuccessful to date. Many of the animals that have been born suffer from health complications and die prematurely. Still, reproductive cloning has provided scientists with some important lessons. For example, they've learned that specialized cells are not permanently fixed, as previously believed. Dolly was produced from a single adult udder cell that was reprogrammed to generate an entire new organism.

Reproductive cloning often inspires heated ethical debate, especially when it turns to the subject of human cloning. It seems clear that if cloning works in other mammals, it can be made to work in humans. Ever since Dolly the sheep was born, the U.S. government has sought to control reproductive cloning. The scientific consensus is also against it: With a success rate in animals of about one live birth for every 75 attempts, most scientists agree that in its present state, reproductive cloning technology is simply too dangerous to attempt with humans. Still, infertile couples, couples carrying a recessive gene that they don't want to transmit to their offspring, and parents who lose a young child make powerful arguments in favor of it.

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