Board Thread:The Panopticon/@comment-1296654-20200630105828/@comment-6032121-20200630124455

I feel like it really has to be said: there is no hard evidence that Susan's mother(?) was conceived by the William Hartnell incarnation of the Doctor. In Lungbarrow, she was the granddaughter of the Other long before the Other flung himself into a loom and got recorporated into Hartnell. And Cold Fusion, meanwhile, suggests that it was Patience's husband/the Douglas Camfield Morbius Doc who sired Susan's father. Heck, Birth of a Renegade opines that Susan was only ever the First Doctor's adopted granddaughter.

It is one thing that TV: The End of Time and the incidental other story makes the philosophical point that a regeneration of the Doctor can be seen as just "a new man" with the previous incarnation's memory. But I could stand here all day listing instances that literally, explicitly say the various incarnations of the Doctor are "the same man". Just from the Tennant era, it's a pivotal point of The Christmas Invasion when David Tennant is recognised as "absolutely the same man" as Christopher Eccleston.

As for the purely genetic argument — well, firstly, see again the fact that the First Doctor may not actually have been genetically related to Susan at all. Secondly, looms. But also, thirdly, even if we focus on accounts where Susan was born "normally" and where she was born to a child of the First Doctor… Time Lord DNA is weird and we do not know how it works. The Tenth Doctor's "daughter" comes out looking more like the Fifth. It's not unreasonable to propose that a Time Lord's DNA remains constant through regeneration, and contains the potential of all their possible regenerations. Or something. We don't know.

Tl;dr, the only reasonable parameter for relatives of Time Lord is self-determination. If Susan Foreman calls any Doctor "her grandfather", or if any Doctor calls her "granddaughter", then we should take them at their words. They're the Gallifreyans and they know how their family vocabulary works better than we do.

Ergo,if Susan calls Eight "Grandfather", we have no business disputing that statement.