User blog comment:The Crimson Eagle/Crimson Poll XVII/@comment-4999424-20120903183135/@comment-4564231-20120907161655

Theoretically, it's not possible to have 100% 1st Civ DNA and still be human. For cross-breeding to work at all they have to have the same number of chromosomes. The bloodline would get diluted as more humans joined the bloodline, but there's usually a 1 in 4 chance that some 1st Civ genes are transferred. Desmond's just lucky to have both parents with moderate gene concentration and even more lucky to end up with a higher concentration. According to William, one person in a million has the same concentration as Desmond.