Meta-Crisis Doctor

The Clone Doctor was created as a result of regenerative energies present within The Doctor's severed hand.

When Donna Noble touched the casing of the severed Hand, it broke as it took in the Human DNA and created a Time Lord/Human hybrid that possessed one heart and aged as a Human did. The new Doctor also contained parts of Donna's personality and mannerisms but mixed with his own intelligence.

The new Doctor had caused the genocide of the Daleks, something the original Doctor disapproves, even when it comes to the Daleks. Because of this, the original Doctor decides he must stay on the parallel earth known as Pete's World. The new Doctor, being born at a time of war, is almost like his ninth incarnation. The original Doctor tells Rose that she is the only one who can make him a better man, as she already has before.

The new Doctor has one major difference than his original self. As he is part human, he only has one heart and can age and die without the ability to regenarate. Being part human also gives the new Doctor one advantage over the original... He has the ability to say and do things that the full Time Lord cannot. He tells Rose that he IS the Doctor, with the same thoughts and the same memories and feels the same things, and that as a part human wants to spend his life with her and grow old together. When asked by Rose what he would have said to her that day on the beach of Bad Wolf Bay, he whispers the words in her ear and they kiss passionately. The original Doctor and Donna depart and Rose and the Clone Doctor are last seen holding hands and looking at each other. (DW: Journey's End)

Behind the scenes

 * Although he the same appearance, memories, and basic personality of the Tenth Doctor, the new Doctor also exhibits several personality changes, in particular based upon that of Donna Noble. It is left deliberately ambiguous as to whether the new Doctor is actually a new incarnation of the original Doctor (if not strictly speaking a new regeneration) or a copy.
 * In the Doctor Who Confidential installment, "End of an Era", executive producer Julie Gardner confirms that the new Doctor does indeed say "I love you" when he whispers in Rose's ear.