It was 10 years ago this week, on a warm July night, that a newborn lamb with an unique pedigree took her first breath in a small shed tucked in the Scottish hills a few miles south of Edinburgh. From the outside, she looked no different from thousands of other sheep born each summer on surrounding farms. But Dolly, as the world soon came to realize, was no ordinary lamb. She was cloned from a single mammary cell of an adult ewe, overturning long-held scientific dogma that had declared such a thing biologically impossible. Her birth set off a race in laboratories around the world to duplicate the breakthrough. It also raised the specter--however distant--of human cloning.
A decade later, scientists are starting to come to grips with just how different Dolly was. Dozens of animals have been cloned since that first little lamb--mice, cats, cows, pigs, horses and, most recently, a dog--and it's becoming increasingly clear that they are all, in one way or another, defective.
It's tempting to think of clones as perfect carbon copies of the original--down to every hair and quirk of temperament. It turns out, though, that there are various degrees of genetic replication. That may come as a rude shock to people who have paid thousands of dollars to clone a pet cat only to discover that their new kitten looks and behaves nothing like their beloved pet--with a different-color coat of fur, perhaps, or a completely different attitude toward its human hosts.
And these are just the obvious differences. Not only are clones separated from the original template by time--in Dolly's case, six years--but they are also the product of an unnatural molecular mechanism that turns out not to be very good at making identical copies. In fact, the process can embed small flaws in the genomes of clones that scientists are only now discovering. The more scientists have learned about the inner workings of the procedure that created Dolly, the more they are amazed that she survived at all.
"We are still surprised that cloning works," says Ian Wilmut, the embryologist who led the team that created Dolly. Ten years and 15 mammalian species later, the efficiency of the process is no better than it was at Dolly's birth: only 2% to 5% of the eggs that start out as clones end up as live animals. For each clone born, hundreds of others never make it past their first days and weeks, the victims of defects in development too severe to allow them to survive.
Clones are vulnerable throughout the cloning process, from their first days in a culture dish to their final moments in the womb to their first weeks after birth. (By contrast, embryos created by in vitro fertilization, which also start out in a petri dish, are pretty much home free if they make it past the first month in the womb.) Dolly, in fact, was the sole survivor of 277 cloning attempts. Clones, as the scientists who make them are fond of saying, are the exception rather than the rule.