User blog comment:D. Cello/Assassinews 10/11 -- Desmond's Ancestry Revealed/@comment-80.42.191.164-20111118200021

Both Ezio and Altair are descendants of Those Who Came Before, but not necessarily the same ones. Altair, in reality, looks very little like Desmond; it's the Animus that made them appear similar in the first game. I do think, however, that Ezio and Desmond look more similar, since there's mention of the first Animus having a primitive avatar program in Revelations, which was fixed in Rebecca's Animus. Also, in Revelations the faces of Altair and Desmond have been altered, Desmond only subtly but Altair to reflect that the first version of him we see is just a projection of the old Animus.

Now, with that explanation out of the way, I think Desmond's father is a descendant of Ezio, while his mother is a descendant of Altair, possibly the only one til Desmond was born. Desmond brings those two lineages together, which is why he's so important. I think he's a major factor in the modern Assassin/Templar war, as they both want him on their side.