Why are some animals harder to clone than others?

Cloning Pictures Cloning isn't a quick process that spits out duplicates like a copy machine. See more cloning pictures.
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­R­eproductive cloning, in which you create an exact ge­netic duplicate of an organism, isn't like photocopying pages in a book. The cloning process doesn't simply spit out replicas ad nauseum. Nevertheless, that concept seems to pervade the public's perception of the technology, fueling fears of clone armies and herds of cloned super-animals in the future.

Currently, there have been 15 documented species successfully cloned: cat, dog, horse, mule, sheep, guar, bull, mouse, goat, pig, horse, cow, deer, banteng and rabbit. Each time scientists successfully clone a new animal species, p­eople often recoil, viewing it as another step closer to human cloning. But worries about generating cloned people like snacks out of a vending machine are largely unfounded since, for one thing, many countries ban or strictly regulate research on human reproductive cloning. In fact, the United Nations attempted to pass a global ban on human reproductive cloning in 2005. Secondly, cloning is often a tedious process with a success rate less than 5 percent. Remember Dolly the sheep? That revolutionary animal clone followed 277 unsuccessful attempts.

­Cloning proponents argue that this science offers invaluable knowledge about how the human body works and the potential of extracting stem cells from cloned embryos. Yet after more than a decade following the cloning of Dolly, the odds of success have barely improved. This method of reproductive cloning has remained largely unchanged, and many scientists have acknowledged that cloning for reproductive purposes is rarely a practical venture [source: Kiem].

On top of that, animals' reproductive patterns can compound the challenges of cloning technology. Case in point: hard-to-clone animals like dogs. Who knew that man's best friend would also be one of the most difficult mammals to clone? The first cloned dog, an Afghan hound named Snuppy, took 1,095 failed embryos to get a winner. Researchers of the Snuppy project, at Seoul National University, used 123 surrogate dogs before one could carry a cloned puppy embryo to term in 2005 [source: Gorner].

­Scientists have tried for years to clone chickens and monkeys as well, with little progress. These cloning hurdles highlight the fact that cloning isn't a one-size-fits-all pattern for asexual reproduction. Just like your eyeglasses might not enhance a friend's vision, scientists have had to make adjustments with cloning procedures to fit the genetic nuances of different species.