Saturday, December 20, 2008

Why mtDNA and Y chromosomes don't necessarily say much about who your ancestors were

Genetic testing is getting to be a big thing now, if underground, with people from the U.S. looking for answers as to their ultimate ethnic origins in testing that looks at the mitochondrial DNA inherited from mothers and Y chromosomes inherited from fathers. But the holy grail of sorting all this out isn't what it appears to be. The theory is that since mitochondrial DNA doesn't change you can trace it back to your mother's mother's mother, ad infinitum, finding where you're from, and that Y chromosomes in men do the same thing, going from father to father to father. All of the genes are passed on like this, but there's no guarantee that the genes you ultimately find will belong to the ethnic group that you recognize you're a part of, if you recognize one.

The reason is really simple. Say way back one population absorbed another one. Men of one group married women of the other, and maybe it happened in reverse, but the culture of the first group became the culture for the second group as well. Now you have mitochondrial DNA from two groups present in a single ethnic group today. Now, imagine that this new group migrated west and absorbed yet another group, but this time staying put. Within the mitochondrial DNA you would have strains from all three of these people, and you'd have Y chromosomes from all three of them.

So far so good. But let's say further that the group that was just absorbed has relatives that live all around it that weren't absorbed. If you do mtDNA testing it's likely that you will find that you're related to these people, even though your parents and parents parents thought of themselves as belonging to the group that assimilated them.

And, in point of fact, they are still members of that ethnic group, because of the unreliability of matching mitochondrial DNA exactly with ethnic origin. It's because they grew up with or had ancestors who belonged to this culture that they're part of it, not because a DNA test proved it.

So mtDNA tests and Y chromosome tests may not tell you what you want to know: they might just tell you who married into your family as either a mother or a father hundreds of years ago.

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