The Generated Anomaly
Quick version: In the canon episode "The Doctor's Daughter", Jenny was created by using the Doctor's (the original's) genes in a machine that spliced the two types of donor genes it required into a suitably random-ish pattern that created the girl-soldier that would be named Jenny by Donna.
The information on that episode, and Jenny in canon, can be found here.
Interesting tidbit about the actress who played Jenny: She's actually dating/engaged to David Tennant, who played the Tenth Doctor and they have a daughter. Georgia is also the daughter of Peter Davison who portrayed the Fifth Doctor. (Must have been an interesting "Meet the 'rents" night" the day she brought him home...)
The information on that episode, and Jenny in canon, can be found here.
Interesting tidbit about the actress who played Jenny: She's actually dating/engaged to David Tennant, who played the Tenth Doctor and they have a daughter. Georgia is also the daughter of Peter Davison who portrayed the Fifth Doctor. (Must have been an interesting "Meet the 'rents" night" the day she brought him home...)
Jenny's Return
Jenny returned to our version of Who-verse when the ship she on passed by the Earth and she decided to end her working contract, semi-amicably, with them and use an escape pod to jettison herself to Earth.
There she meant Anthony Tyler and Sarah Jane Smith, who instantly recognized that something wasn't quite right with the human looking young woman. When Jenny asked about the Doctor, Sarah Jane and Tony took Jenny to Cardiff where she met the others, and eventually the new Doctor herself who had the sad duty of telling her that while, yes, she had found Donna again but that her 'father', the Doctor she remembered, was long since passed.
Jenny was devastated but accepted the Doctor's offer to travel with her and Donna.
There she meant Anthony Tyler and Sarah Jane Smith, who instantly recognized that something wasn't quite right with the human looking young woman. When Jenny asked about the Doctor, Sarah Jane and Tony took Jenny to Cardiff where she met the others, and eventually the new Doctor herself who had the sad duty of telling her that while, yes, she had found Donna again but that her 'father', the Doctor she remembered, was long since passed.
Jenny was devastated but accepted the Doctor's offer to travel with her and Donna.