Spatial genetic multiplicity

Spatial genetic multiplicity was what the Doctor used to explain how Gwen Cooper, a friend of his former companion, Jack Harkness, and Gwyneth, a servant he and Rose encountered in 1869, (DW: The Unquiet Dead) looked identical. (DW: Journey's End)

Behind the Scenes

 * Russell T Davies addressed spatial genetic multiplicity, explaining it as "It's not familial as we understand it. There's no blood tie. Spatial genetic multiplicity means an echo and repetition of physical traits across a Time Rift."