Spatial genetic multiplicity

Spatial genetic multiplicity was what the Tenth Doctor used to explain how Gwen Cooper and Gwyneth looked similar. (TV: The Unquiet Dead, Journey's End)

Behind the scenes

 * Russell T Davies addressed spatial genetic multiplicity, explaining it as "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."
 * Spatial genetic multiplicity could apply in many instances throughout Doctor Who, as numerous actors have returned to the show to play different characters. Notable examples include Peter Purves playing both Morton Dill and Steven Taylor in The Chase, or Jacqueline Hill (best known as Barbara Wright) playing Lexa in Meglos. For the most part, this is not usually of plot significance. One gag in The Daleks' Master Plan ("The Feast of Steven") has the First Doctor remark that Reg Pritchard's character looks similar to the character he played in The Crusade.
 * More intentional cases of lookalike characters have been part of stories in non-visual media, matching Ian Chesterton with Robin Hood, (PROSE: The Thief of Sherwood) and Polly Wright with both Tatiana Kregki (AUDIO: The Night Witches) and Maid Marian, (PROSE: The Thief of Sherwood, That Time I Nearly Destroyed the World Whilst Looking for a Dress) thought whether spatial genetic multiplicity is at play here is not brought up.