How accurate are DNA analyses of your ancestry?

Factual Questions
1

I just read this comment in a recent thread on DNA and the Black Dahlia case:

"#16 Today, 03:42 PM
Chronos
Charter Member Join Date: Jan 2000
Location: The Land of Cleves
Posts: 51,946

Quote:
FtGKid2 did the 23AndMe thing a year ago and got a match in their public DNA database for one of my 2nd cousins. So I don’t think the fall-off for matching over the generations is all that bad.

You’re always exactly 50% related to your parents or your children, but to your siblings, it’s only 50% on average: It could in principle be anywhere from 0% to 100% (but more likely somewhere in between). A similar principle applies to cousins. So that cousin of yours, just by luck of the meiotic dice, might happen to be more related to you than the average second cousin."

I wonder exactly how accurate those DNA tests that allegedly find your “hidden ancestry” are – the ones various Hollywood types are agog over when they discover they’re, say, 12% Sephardic Jew from Spain and 8% Swiss.

Are these just general guesses based on genetic markers or do they really pan out as real people when compared with known family background? Do they represent real ancestors, or just a chance DNA squiggle you picked up along the way?

And while I’m at it, if I have a great grandfather who was a Jew, is it possible that I might carry the Tay-Sachs gene so many generations later?

2

IANAGeneticist, but I think the answer is that the number is in fact precise, and has a precise meaning, but the interpretation of the number in terms of “how alike you are to genetic group X” is much trickier.

If I understand correctly, what these analyses generally do is look for a collection of SNPs in your genome (Wikidefinition of SNP here: Single-nucleotide polymorphism - Wikipedia). So they are NOT, actually, comparing your entire genome – all your DNA – to some stored database of genomes, and they’re also not comparing functional genes, for the most part, meaning we see here that you have/don’t have this gene for a particularly big honking nose that we find in Arabs of this-and-such historical tribe, or you have/don’t have this gene for Nordic blue-eyed blondness.

What they are doing is finding places in the genome, often enough noncoding places (meaning parts of the DNA that do NOT code for an actual protein, but are involved in regulation, or have no purpose at all), where there happens to be a recognized single-nucleotide flip. They look at the pattern of such flips you have, and compare this to a database of flips, that says, for example, people we’ve tested from Eastern Poland have flips a, b and c with probability 99%, 2% and 88%, while for people from Brazil it’s more like 1%, 100% and 70%, and since you’ve got a and c we conclude you’re probably more Polish than Brazilian. That’s a gross oversimplification without question and involves a bunch of guesswork, but the point is that I think it’s possible to construct a pretty precise number, something like saying you have a 12.5% chance of being a member of this particular cohort.

But what that number means is much trickier. Keep in mind almost all of our DNA is not only identical, it is identical to that of apes, dogs, and dandelions. It’s only a small amount that distinguishes humans from baboons, and an even smaller amount that distinguishes races and families. We know very little about what differences are “critical,” in the sense that they make the macroscopic observable differences about which we care. There’s no labeling on the DNA that tells us this – all we know is that this DNA is similar in this ethnic group, and different in this other. Does it matter? Unless we know what that stretch of DNA does, we don’t know. Maybe it codes for some difference we consider extremely important. Or maybe it codes for nothing at all and this variation in the DNA produces no variation between the groups at all.

I think the assumption is that similarity in the collection of SNPs is probably *generally *correlated with similarity in the characteristics we consider important in discriminating between tribal groups. So if your SNP distribution has only a 12.5% chance of being drawn from the distributions of distributions we’ve measured in actual Ugandans, we conclude you have a low probability of an Ugandan ancestors – but we *can’t * say you have a 12.5%, or indeed put any really precise number on the conclusion. Or we can say you have a low probability of having Ugandan genetic characteristics, but we can’t specify whether you do or do not have any particular macroscopic characteristic we identify as important in deciding who’s an Ugandan.

Hopefully a more modern-educated bioinformatics guy will come along and clean that up, or set me straight if I’ve wandered into error.

3

I’m not sure what my post has to do with any of that, either.

4

In my own experience, very acurate.

I have identified almost 10 real live people that are DNA matches to me, up to 5th cousin level, as true relatives of mine. I have spent years building an extensive family tree of both every ancestor and descendant I could prove.

I say almost 10, because 2 individuals would not reveal their exact identity, but did look at my tree. One confirmed they were in my tree and the other confirmed one parents blood relationship to me but refused to identify their parent or to say if they personally were in my tree.

Of the other 8, some were already in my tree and some were not, but I had enough information to easily place them and then fill in the missing people and information to add them.

Of course I still have another 1,000+ matches I am working on. A lot of your success rate will depend on the level of cooperation you get from your matches.

5

Inspiration; the occasion for a question on DNA “inheritance” services that’s been in mind.

I’m a layman, as the form of the question will attest. Had I had seen a clear and direct connection to the original thread, I would have just added it to the discussion there.

“Carl Pham” is addressing what I wanted to know quite well here. I’m also interested in what “LurkerInNJ” is saying; I’m assuming that he used one of these DNA analysis services, which seem to have become part of popular culture at the moment – hence my question – and has set out to document actual results.

If there is some vital point I’m missing here please clarify it; as I said, I’m strictly a layman with a layman’s question.

6

I’d agree with Carl Pham. There’s no reason to believe that the DNA analysis itself is inaccurate. These days, it’s quite easy to do with the right equipment and expertise. The sticky bit is interpreting all that data. You need to have a very robust database of SNP patterns found worldwide to come to any meaningful conclusions about what patterns are associated with what regions of the world, and it is true that every generation that passes is going to muddle the picture somewhat. I suspect that somewhere in the fine print they have some sort of explanation of what sources they’re using and the likely error rates are, and all that jazz.

7

I’m FEMALE :wink:

I’ve used all three major testing services, but had put in many years of genealogical research before DNA testing for ancestry purposes became something the average person could afford, so I had a fairly well proven background on paper.

23andMe does a good job of explaining how they reached their conclusion, and as the years pass and the process becomes more and more refined, your results do change somewhat. For example, years ago my ancestry composition included a small portion of India, which later turned into Guyana (which made sense because many people in Guyana are descended from people originally from India), which has now suddenly mutated into Yakut. No idea how or why, and from past experience, I would expect that to change eventually. Their overall results generally supported my British/Republic of Ireland/German/Finnish/Swedish background, with a few pinches of other things mixed in. One I strongly suspected, the others were fun to consider.

Ancestry lumped me into the 99% British/Irish category. I know for a fact I’m not, and not just because research tells me so. There are people in my tree who are still alive that were born in Sweden and Finland, and they were in the second group of family that came here to join the first. No German either, and that background is undisputed and also supported by three identified DNA matches in my tree.

8

[quote=“LurkerInNJ, post:7, topic:682278”]

I’ve used all three major testing services, but had put in many years of genealogical research before DNA testing for ancestry purposes became something the average person could afford, so I had a fairly well proven background on paper…

23andMe does a good job of explaining how they reached their conclusion,suspected, the others were fun to consider…

Ancestry lumped me into the 99% British/Irish category. … [end quote]

LurkerInNJ, by Ancestry, do you mean the test offered by Ancestry.com? I had heard of 23andMe, but not sure of other companies that do this testing. Do you have a preference? I’m pretty sure I’d be lumped into the 99% group as well (white bread all the way back for the most part), but thought my mother might like to find out if the stories she’s always heard about her grandmother being Native American were true. I’d prefer to have her tested than me, being a generation closer to the source. The purpose is not to claim any tribal heritage, just curiosity, and possible help in finding anything on the woman, as we have hit a dead end. Any advice?

9

My experience has been very different from LurkerInNJ. My spouse and I have run 1 Y chromosome test and 2 mitochondrial DNA tests and, despite have fairly decent family trees going back 4-5 generations, have never found any previously unknown relatives through DNA testing. All we found out is that our ancestors seem to have been who we thought they were.

So we did the FamilyFinder (or whatever they call it) which says both of us are about 90% northern European and 10% Middle Eastern which sounds interesting (was great-grandfather Paddy McGillicuddy born Izzy Cohen?) until we learned that lots of northern Europeans show some Middle Eastern genes, most likely not from one great grandparent but from two great, great grandparents, or 4 great, great, great, grandparents, etc. (or some combination of the above).

So, all in all we figured that we had found no lost relations and learned nothing from our $1000 investment, except that our ancestors probably were who we thought they were, until we looked at the list of other people who had had their DNA tested and had their results in the database and had similarities to ours. While we found no relatives, the ethnicities of the surnames of people in the 2nd to 5th cousin-equivalent category for me matched my ethnicities. But my spouse’s 2nd-5th cousin-equivalents had a mix of European names but half were Ashkenazi, despite the fact that my spouses only only known Jewish ancestor was a great-grandmother who would be expected contribute only a small proportion of genes (like 12.5%). Of course, it could be that my spouse’s Ashkenazi relatives are just much more likely than the non-Ashkenazi relatives to have their DNA tested and listed in the database, or it could be one or more of my spouse’s ancestors were not exactly who we thought they were.

10

LurkerInNJ, What do you mean by “Ancestry lumped me into the 99% British/Irish category.” Was this an mtDNA result or “Family Finder” result?