User blog comment:KillHil/Assassin’s Creed III Ventures into the Wild/@comment-69.146.244.153-20120407200709/@comment-4829046-20120409064811
You forgot one CRUCIAL fact. The blood of the first Civilization has been passed down from Altair all the way to Desmond. Desmond's family has the rare ability of eagle vision because THEY ARE RELATED BY BLOOD not through marriage.
The ones who have eagle vision in the games are Ezio and Altair, both are ancestors of Desmond. For Desmond to get the rare genetic code means that it had been passed down for centuries.
Here from the wiki; Desmond Miles came from many different lines of Assassins through BOTH sides of his family. As a result, Desmond had a higher concentration of First Civilization genes in his DNA than most other humans, allowing him to properly wield the Pieces of Eden,[1] which the First Civilization had created so that only their own race could wield them.[5]