Well, technically, immortality is a permanent bond associated with a very powerful True pattern.
Observe, a Blood Oath can be taken, based on the Name of your Oath. When you defile the Pattern of your Oath, the link you've made with it will kill you. Much in the same way that a Shedim bonds with a corpse to prevent its decay or a free spirit can continually return from a Metaplane after being disrupted. These bonds are in essence a link to their Metaplane, and their True Name is actually the "pointer" to the bond. Destroy the bond and the spirit becomes "mortal" and will die because it has no physical form to fall back on.
Immortal Elves are bonded in some way to a permanent thing or ideal. You can become bonded to the Idea of Hatred and become immortal. However, such a bond does not come without a price, if you sway from the Definition of the Name you are bonded to, you WILL die, horribly so, in fact. Also, these bonds require huge amounts of Karma to create and so, are a very very rare thing indeed.
How does genetics come into play then? Simple, if Karma represents your experience and knowledge and actions alter your memory RNA so that you can remember your experiences, why is it impossible to say that memory also affects the secondary and tertiary structures in your DNA. The immortal gene activates after a drastic amount of time is spent bonding with the True Pattern of your choice.
Allow me to reiterate that the strongest form of Magic in the SR/ED universe is the ability to alter True Patterns. This is in essence what "Solomon's Key" is really; the ability to modify the universe on a Named basis. The most powerful of rituals involve the changing of these patterns. An example would explain it better:
Say you have a toaster, everyone else knows it is a toaster, you put bread in, it cooks it, and makes toast. Given enough power and energy and a knowledge of the True Pattern of the Toaster, you can make it something else entirely, a light bulb, a frozen dessert, etc. You can even change the apparent history of the object through space and time. With enough power, you could rewrite the history of World War II or Vietnam. However, such power would be impossible to must at this time without Blood Magic, so no one can really do that.
