Doctor Who
Thirteen actors have headlined the series as the Doctor. The transition from one actor to another is written into the plot of the show with the concept of regeneration into a new incarnation, a plot device in which a Time Lord "transforms" into a new body when the current one is too badly harmed to heal normally. Each actor's portrayal is unique, but all represent stages in the life of the same character. Together, they form a single lifetime with a single narrative. The time-travelling feature of the plot means that different incarnations of the Doctor occasionally meet. The Doctor is currently portrayed by Jodie Whittaker, who took on the role after Peter Capaldi's exit in the 2017 Christmas special "Twice Upon a Time".
Doctor Who
Although the idea of casting a woman as the Doctor had been suggested by the show's writers several times, including by Newman in 1986 and Davies in 2008, until 2017, all official depictions were played by men. Jodie Whittaker took over the role as the Thirteenth Doctor at the end of the 2017 Christmas special, and is the first woman to be cast as the character. Whittaker had previously starred in television series such as "Return to Cranford", "Broadchurch" alongside David Tennant (Tenth Doctor) and the dystopian anthology "Black Mirror". The show introduced the Time Lords' ability to change sexes on regeneration in earlier episodes, first in dialogue, then with Michelle Gomez's version of The Master.