Because identical twins often look almost exactly the same and share the same DNA, people often make the mistake of assuming that identical twins are actually "clones" of one another. In the strictest scientific sense, this is not true.
Identical twins originate from one egg that has been fertilized by one sperm. Sometime early in the cell division process, for reasons still unknown, this fertilized egg splits into two distinct embryos. Though these embryos are genetically identical (sharing the same DNA), they both originated from the fertilization of an ova.
The origin of a clone, at least using the current technology, does not involve sperm cells at all. In the few successful attempts at cloning animals, scientists have removed the nucleus of an egg cell from one animal (Animal A) and replaced the nucleus with the nucleus from a cell from another animal (Animal B). The resulting embryo is then implanted into the uterus of a female animal of that species (Animal C). When the clone is born, it is a genetic copy of Animal B.
There is some evidence that suggests that some genetic degradation takes place when clones are made; perhaps the clone does not live as long, or is not born as healthy as the original animal. On the other hand, there is no evidence at all that either twin in an identical twin pair suffers any sort of genetic damage when the original egg splits into two individuals.
Currently, cloning is a very difficult, expensive, painstaking process that has yielded very few results and little usable technology; the leader in the field, a scientist in Korea, was recently outed in his fraudulent claims of success in cloning stem cells. Much of the current controversy surrounding cloning centers on the possibility that scientists will be successful in the cloning of humans. Many people are concerned about the ethics of such a development, because it would mean that anyone could conceivably create a clone of him or herself instead of just having a baby the "old fashioned way." If humans could be cloned, there is also the worry that embryos would be created simply to grow organs or other necessities for the rest of the human race. Much science fiction has been based on this concept.
In summary, identical twins are not clones. They are distinct individuals who originated from the meeting of one egg and one sperm. Clones are genetically engineered organisms. If one really wanted to split hairs, identical twins more than likely share more similarities than clones, because clones depend on a "host" egg from Animal A, which contains its own properties, separate from that of the original animal (Animal B)--and these properties' effects upon the implanted nucleus is unknown at this time. Additionally, clones do not share a womb with the original animal, and so are affected by Animal C in ways that are not fully understood.
Identical twins, though they share DNA, are unique individuals. Comparing them to clones is not only unfair, but counterproductive to the development of individuation of each twin. While it is important to explore the ethics of cloning humans, the fact remains that cloning a human being has not been done successfully; if and when scientists figure out how to clone humans, it is doubtful that this process will be easy, affordable, or ethical. In our explorations of the ethics of cloning, we should not bend to the temptation of comparing clones to twins. Twins occur in nature, in many species of animals--clones are a creation of human beings, and as such, belong in a separate category.
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