Comments made during Podshock 326 about The Doctor's first televised companion, Susan, got me to thinking about how little we know of Gallifreyan culture. While what we do know may seem lacking or vague, I have no problem accepting it as sufficient, complete and canon.
Sketchy details are part of what makes it possible for Doctor Who to evolve. Case in point, the Chameleon Arch. Less is known about it than is known of Susan's past, yet how many discussions or debates have you been part of or learned about after the Arch essentially fell from above, worked its magic and was accepted as though it's been there all along? Fandom moved on.
Back to what sparked this collection of thoughts about the origins of Susan and conjecture that The Doctor's daughter (Jenny) is Susan's mother, for starters that line of thinking seems to either contradict or render null and void the 10th Doctor's comments to Donna in The Doctor's Daughter...
DOCTOR: Donna, I've been a father before.
DOCTOR: I lost all that a long time ago, along with everything else.
DONNA: I'm sorry. I didn't know. Why didn't you tell me? You talk all the time, but you don't say anything.
DOCTOR: I know. I'm just. When I look at her now, I can see them. The hole they left, all the pain that filled it. I just don't know if I can face that every day.
The Doctor was a father before, long before Donna was born or Jenny was created however little is known about 'family life' on Gallifrey. Nothing has been revealed to indicate the manner in which one becomes a parent, whether it resembles what we humans experience or perhaps somehow resembles the creation of Jenny or for that matter bears no resemblance to either.
No matter how Gallifreyans procreate, in a canonical sense, Jenny would have to travel back in time to The Doctor's first incarnation to become Susan's mother / creator.
While not implausible provided one accepts time travel as possible, a key precept of Doctor Who, Jenny had to first be created before she could become Susan's mother / creator in the past. Doesn't that make Jenny both immortal and ephemeral? Can she be both?
'Jenny The Ephemeral' because as Susan's mother / creator she is lost to The Doctor. Such a loss could have been The Doctor's doing brought on by an act as simple as running away from Gallifrey (with his granddaughter in tow) in a malfunctioning TARDIS. On the other hand The Doctor's loss of family may have been none of his doing. Believed dead (for the sake of argument) or known dead (in the event of cataclysmic or similar events such as a Time War where an entire planet or civilization is known to have been destroyed, his kin may have simply been in the wrong place at the wrong time.
In any event, The Doctor's family went completely missing from any and all of his incarnations and timelines no matter how much wibble you put in the wobble.
'Jenny The Immortal' because while she is created just once, she lives on forever in a canonical loop which began with The Doctor's Daughter. This chronic hysteresis extends back to the place in time of An Unearthly Child, repeats every event from then until The Doctor's Daughter only to do so again and again for all of eternity, which to my way of thinking makes Jenny an eternal being if not immortal.
If on the other hand we simply accept the onscreen relationship between the 1st Doctor and Susan for how it is set forth, as a relationship between a Grandfather and his Granddaughter, then anything we read into that relationship is just another pattern that isn't there. 'I love humans, always seeing patterns in things that aren't there.'