Well, that's a logical explaination that covers most ground, I say. Genetically speaking, that could be the case if only Shiki is the one to be born, in all human history, with this dormant MEoDP gene; but leaves the chance that somebody else that was born in the past have that same gene, leaving also the chance that, theoretically speaking, almost anybody could have it.
Let's imagine a scenario taking place some time in the past where a subject is born with that gene. Since it's a gene, as you say, it could be passed on to it's offsprings, right? and the same would go for the offsprings' offsprings and on and on. That would leave generations of a family having that same dormant gene. If we consider the family to be one to reproduce solely through inbreeding (as the Nanaya), we could say that this could be a closed environment where only this family is the one with such a gene, but that is only if you could isolate the very first subject to carry it.
If, after a number generations, we move the family to an open environment, the subjects carrying this gene would be able to breed with others who don't carry it. This would cause a boom with children carrying the dormant MEoDP, if we move a bit more generations into the future, then a load of children would be potential world-destroyers, if they have a near death-experience
Even considering that a number of the children born to parents who have said gene, have it as a recessive one or don't have it at all, there could still be a lot of others to have it. With modern medicine, stories of people who almost died but "made it back" are increasing in number, it would also mean that people with active MEoDP would increase.
In other words, if there was someone in the past born with this gene, be it an entire family or just one single person, there would already be more than one* person with this ability.
If it was a gene, in order to still be considered "special" (as in, highly rare, near to nobody has ever had it) it would have to be a very, very
peculiar one. The kind to just pop-up unadvised in very counted subjects between strectched lenghts of time only to dissappear in future generations.
If the gene was capable of being passed on to offsprings, then that boom would have ocurred years ago. Since we don't see such a scenario, either:
a) That gene, if it is
a gene, would have to be the kind that show up and then vanish.
b) Subjects who have this gene
are sterile, thus aren't capable of breeding, succesfully killing any chance for the gene to be passed on.**
The conclusion would be that Shiki's daughter or son (take your pick) wouldn't have any more chance to develop MEoDP than, say, the kid next-door, the secretary working her ass off in an office or the starving children in Africa (who subsequently have more near-death experiences, rendering an army of african children capable of slicing the world!
* I'm merely counting it as one
persone to have it, since both T. Shiki and R. Shiki are in different universes.
leaving only possibility a)