A plotbunny about the ATA gene.
Ok, so I'm using part of this in the-story-thats-taking-me-forever-to-write, but I went on with the idea and I'm not going to write this but...
Jack O'Neill is pretty much not-human. His (approx) 17 Y.O. self was changed by an Ascended Ancient when his 40-something Y.O. self went back in time to 1969 and the Ascended Ancient noticed that 40yo Jack had the super strong ATA gene and 17yo Jack didn't and figured that since it was already done (apparently) it would be alright to change 17yo Jack's DNA to mostly Ancient and it wouldn't be counted as "interfering".
Also, whilst in 1969 Jack O'Neill sleeps with 2 different women and they both get pregnant. The two sons that result from said unions are John Sheppard (the elder by about a day) who has the Dominant Command ATA Gene and Cameron Mitchell who also has a v. strong ATA gene.
Neither Jack, John, or Cam know anything about their relations to one another; only John and Cam's mothers suspect, adn John's mom is dead and Cam's isn't about to admit ANYTHING.
(I know Cam doesn't have the ATA gene in Canon, but it's fanfic. Anyway he's still doing rehab when the Expedition leaves so he couldn't have taken BlackMark!John's spot anyway, and by the time Atlantis was back in communication with Earth, You would'a had to have ripped that SG-1 patch off of his cold dead body. (btw, that's paraphrased from someone's rather excellent fic who's name I can't recall at the moment.))
Any way. When Stargate Continuum rolls around and Cameron Mitchell is sent back in time, he is a huge man-slut and ends up impregnating several women over the course of his life, passing along that oh so precious ATA gene.
Carson Beckett and every other person who has the natural ATA gene (with the exception of Jack and John) are the progeny of Cameron Mitchell and his time-travelling man-whore ways that he got from his daddy.
It's been too many generations since the Ancients had children with the humans. Therefore anyone with the ATA gene is the direct decendent of a Ascended-being-altered Jack O'Neill.
When the gene is synthesized from John's gene and given to all the Atlantian crew, the one's that it works for are considered by the Ascended as additional progeny of Jack, via John. This is why the Asgard were so interested in Jack, and why they had a scientist on the ship that was always going to Atlantis. Because for all intents and purposes the ATA gene people, natural and synthetic, were all O'Neills and were the new Ancients.
Ok. Now the premise is hopefully out of my head and I can get back to Jack O'Neill being John Sheppard's dad and John Sheppard being Harry Potter's dad. (3 generations of bastards...)