Genetically, the two are virtually the same. Your identical twin and your clone would each share your exact nuclear DNA-the type of molecules that account for more than 99.9 percent of an individual’s genetic makeup. The remaining genetic material, known as mitochondrial DNA, comes from the egg from which you’ve been conceived.

You and your identical twin would share the same mitochondria because you have the same mother. However, if your clone were created from an unrelated egg, it would have unique mitochondria-and therefore a miniscule amount of genetic difference from you.

“There are 37 genes of mitochondria and 30,000 to 35,000 genes of nuclear DNA in every cell,” says Hessel Bouma III, a professor of biology at Calvin College in Grand Rapids, Michigan. “Even if your clone didn’t come from your maternal lineage (eggs from your mother or sister), there would be only a pretty small genetic difference.”

But beyond genetics, there would be two enormous differences between your twin and your clone. First, there’s age. Contrary to some science fiction imaginations, clones are not born as adults. They start in the womb and become babies like everybody else. If you were 40 when your DNA was used to make a clone, your clone would always be 40 years and nine months younger than you. Your twin, on the other hand, will always be virtually the same age as you.

Secondly comes experience. Your twin and your clone would grow up in entirely different environments, amid unique times and circumstances.

“Look at it this way: If you had two identical computers and you sat down two people to program them, right away each computer would have a completely different set of functions and preferences,” says Bouma. “That’s how it is with human development: Your genetics offer you a pretty clean slate, and your environment influences you enormously from there.”