So, Jean Grey, in the alternate timeline survives long enough to give birth to Rachel Summer/Grey. Is this explored somewhere? Did the Dark Phoenix events just never happen? I suppose, more specifically, what are the changes that cause the Days of Future Past timeline to occur?
Born in a world where the Phoenix Force merged with Marvel Girl, instead of replacing her,
A varient on this page says:
According to Rachel, Jean Grey from her reality mirrored the modern era Jean Grey, but when this Jean was captured by Sentinels and piloted a spacecraft back to Earth, the Phoenix Force bonded with her, instead of placing her in a cocoon and replacing her.
As a result, Phoenix did not commit suicide after the Sh'ar duel of honor (as she had in the modern era). Instead, Jean married Scott and had a single child, whom they named Rachel and who inherited Jean's psychic powers.