Doctor Who season 12 shattered 50 years of continuity with its revelations about the Doctor’s true heritage. Jodie Whittaker’s Thirteenth Doctor discovered that she was not just a Time Lord, but the first Time Lord. Once a lost child from another dimension who fell through a wormhole known as the Boundary, she was experimented on by the early Gallifreyans, who spliced her unique ability to regenerate into their own DNA.
This changed everything we and the Doctor herself understood about who she is. For one, we now know she’s not biologically a Time Lord. Without knowing what she truly is, let’s just use Timeless Child to refer to her species. She’s actually unique in the universe, then. Well, except for one other being who shares her genetic make-up. The Doctor’s own daughter, Jenny, as introduced in the David Tennant episode “The Doctor’s Daughter” (2008).
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In that story, the Doctor landed on the planet Messaline, which was caught in a war between humans and the fish-like Hath. Both sides used cloning machines to instantly generate new soldiers. The Doctor’s DNA was harvested upon his arrival, creating Jenny. We soon learned that she shared the Doctor’s binary vascular system – AKA she had two hearts. In a tragic twist, though, she was killed saving her father’s life. He left believing her dead, but the audience then saw her regenerate – though keep her form – and go off to have new adventures. As depicted in Big Finish’s Jenny – The Doctor’s Daughter audio dramas.
The episode labels Jenny a Time Lord, but we now realize that, because she comes directly from the Doctor’s DNA, she’s really another Timeless Child, the only other person in this dimension who belongs to the Doctor‘s actual species. It’s tragic, then, that she doesn’t even know that her daughter is still alive.