The Cohen Gene, the Samaritans
The Y-chromosome, besides for the genes determining maleness, consists almost entirely of non-coding DNA, that is it doesn't define your body much more than defining your [male] sex (yes people have a sex, words have a gender). Thus it tends to accumulate mutations as this won't effect the carrying organism detrimentally in most cases. It is passed from father to son without recombination. The genetic information on a Y chromosome of any man alive today is basically the same as that of his ancient male ancestors, except for these rare mutations.
Using this information, a common gene has been detected to be carried by all cohanim, priests, descended from Aharon haCohen, the first High Priest and brother of Moses.
It is interesting that cohens from all over the Jewish world, Sephardim, Ashkenazim and Yemenites, carry this gene. Over the generations, we have accepted someone who says he is a cohen to be such on the basis of hazakah, the presumption that their claim is true, given there is no reason or evidence to disprove the claim.
But we can never really be sure. As in other difficult problems in Jewish Law, we say that the real shakeout in this issue will be determined by Eliyahu haNavi, the prophet Elijah, who will return to herald the coming of the Messiah. We seem to attribute many such wonderous skills to Eliyahu. However, the determination of who is a priest amongst us has now been made somewhat easier -- so Elijah, you can come out of hiding now! Your job in this issue is mostly done for you.
The presence of the gene does not solve all the problems. A person who carries the gene may not be a cohen; he may not even be Jewish (halachically). His great-great-great-grandmother may have been raped on the way home from the mikve on a dark night, and if she didn't inform her husband; the son would be accepted by all as a cohen, but of course not be one -- so we can now get rid of those from our list.
But on the other hand, if a cohen married a women he was not allowed to marry by halachic law, such as a divorcee, the offspring is not a cohen, but would carry the paternal gene indicating kehuna. If we find someone who never thought he was a cohen with the gene, I suppose we assume he is the progeny of such a union in his distant past and was disqualified by his community. Or perhaps he was an orphan and the adopting family were not aware of this. But how can we know after all the elapsed generations -- Eliyahu, we do need you!
All this fascinating stuff got me thinking. First, will we find other genetic or scientific knowledge, which we do not as yet possess or know how to use, that will simplify Elijah's other work too? or rather allow us to classify things in halacha that we are unable to presently.
The I started thinking about the Samartians again. If their "priests" possess "the gene", does that validate their claim of being Jews to some extent? But it doesn't make them descendants of the Children of Israel -- their priests could be descended from those whom the King Assyria sent back to Samaria to teach the locals in the ways of the Torah; or perhaps they were "imported" from south of the border in nearby Jerusalem to serve necessary priestly functions. Someone had to be there to receive the agricultural tithes, parts of slaughtered animals that are prescribed by the Torah as gifts of the cohen, the firstborn and other priestly gifts.
Just had another interesting thought. We disqualified the Shomronim from being Jewish, after they had been accepted for hundreds of years because they had strayed; they had an idol in their temple on Mount Grizim. But in today's fight over the cancellation of conversions to Judaism of Rabbi Chaim Druckman, the claim is that we cannot retrospectively cancel conversions. Really? Or did we never really accept the Shomronim as full converts; it was conditional -- we weren't sure -- and then we became sure.
But my mind wonders further. Is the cohen gene the only miraculous DNA we have? What if every one of the twelve (or thirteen) tribes have their own markers? We could use this to determoie which Afghan tribes were ours and which were not (assuming, as is often claimed, that these tribes are the progeny of the Lost Ten Tribes).
Maybe the Khazars had a national, tribal gene too. Are Ashkenaz Jews descended from the Jews expelled from Israel after the period of the Jerusalem Talmud, or are they largely the sons of the converted Khazar nation?
So many possibilities! Can we solve these problems ourselves, or do we need Eliyah and the Moshiach (or is it Mashiach?).