I've heard people say that Desmond was not related to Altair and/or Ezio and that Ezio is not related to Altair. If that's true, then how did he access their memories? I thought that he could only connect with his ancestors' memories. Am I wrong, or just confused?
The story states that the memories are stored in the DNA; thus he must be related to them both in order to have have their DNA within him.
Coincidentally, 1up just ran a feature on the worst of video game science and because of the storing of memories in DNA they mentioned that there is about 1.5GB of information contained within our DNA (both in useful genes and in the filler space).
The article doesn't mention that since half of your DNA comes from your mother and half from your father you'd wind up with some seriously disjointed memories.
Inside this interview with the developers, they tell us how Desmond is related to all of them in some shape or form.
Here is one of the illustrations they used inside the video describing how Ezio is not a descendant of Altair. Basically, they want Desmond to access all of his ancestors memories, and not let Altair or Ezio be able to access others memories.