Are spoilers out? Is it time for me to be offline for once?
Read "The Adventures of Cyclops and Phoenix" Jean spent over a decade in the future raising her son. It came out shortly after the 2 got married in the 90s.
After Rachel/Mother Askani took Cable into the future, they were still unable to treat him. They had assess to Ship who they used to keep him alive. Then they decided to clone Nathan because they weren't sure if they could truly save Nate. Then Apocalypse attacked, killed Mother Askani, and took Stryfe who he believed to be the real Nathan Summers. Using what power she had left, Rachel pulled Scott and Jean into the future where they raised Nathan as their son.
This is one of the reasons why Jean always gets more emotional over Cable than Rachel who she never raised.
Reminds me of this K-Drama I watched, Call Me Mother. There's this heart wrenching scene where a mother tells her adopted daughter that she is her mother, despite what the daughter might say about her being "just adopted". Her lines were something like "I'd spill my blood and give it to you if you really think that'd make us closer."
Thankfully Cable's never ever thought of Jean like that and I'm sure Jean would say something similar. But, yeah, I agree that Cable has Jean's DNA so what's the difference even?