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Why don’t the DNA matches match

Julie Matthews is puzzled by her DNA test results:

Why do none of my Y matches also match my autosomal matches? And why are all the Family Finder matches I’m finding ending up being tenth cousins using a paper trail rather than the promised five generations?

Great questions, Julie — the autosomal tests (Family Finder from Family Tree DNA, Relative Finder from 23andMe and AncestryDNA from Ancestry.com) can be really confusing. Let’s see if we can work through these to help make sense of the results.

First, a bit of basic biology for folks new to DNA testing. Every human being has 46 chromosomes — 23 inherited from each parent. Two of those 46 chromosomes determine gender. If the person has two X chromosomes, she’s female. If the person has one X and one Y chromosome, then he’s male. The 44 chromosomes that don’t determine gender are called the autosomes.

So for a Y-DNA test, only the DNA from the Y chromosome is tested, and it can detect common descendants only in a direct male line: father to son to son.

For the autosomal tests, only the DNA from the autosomes is tested, and it can detect cousins both male and female.

And that, most likely, is one part of the answer to why your family Y-DNA matches (I assume that test was done with a brother, uncle, father or male cousin) don’t also show up in the list of those who match you in the autosomal test: a very large number of people who’ve tested for Y-DNA simply haven’t done the autosomal test, and vice versa.

If you look at your list of Family Finder matches, and look in the area under each person’s name, you’ll see some symbols (for email, for notes and, in some cases, for a gedcom if the person has uploaded one). And in some cases you’ll also see some other information — and it’s that other information you want to focus on because it tells you what other DNA testing the person has done.

Y-DNA followed by a number means this person — always a male — has had his Y-DNA tested; the number tells you how many markers he’s tested — 12, 25, 37, 67 or 111. HVR1 or HVR2 or FMS means the person — male or female — has also had mitochondrial DNA (mtDNA) testing done. The letters tell you which test: HVR1 is the least complete, HVR2 the next best and FMS the gold standard — complete mtDNA testing.

If you don’t see any of those indicators under a person’s name in your Family Finder match list, then that person hasn’t done those tests. So the first thing to look at is whether a Family Finder match has even done the testing you hope will match your results. In my own experience, I’ve found that the vast majority of interesting Y-DNA matches to the men in my family haven’t done autosomal testing — at least, not unless I’ve volunteered to pay for it!

The second thing to keep in mind is that Y-DNA and mtDNA don’t change very much from generation to generation, so testing those types of DNA will give you matches that can be very far back in time. By contrast, autosomal DNA changes and recombines so quickly that its usefulness for genealogy pretty much punks out after only five or six generations. (As you’ve found, you can get matches back beyond that but it’s pure luck that that’ll happen.) And this means that a really good solid Y-DNA match can easily be way too far back to show up in your autosomal test matches.

Of course, if you do happen to have a solid Y-DNA match who has also done autosomal testing and doesn’t match you, that can be a clue that may help you figure out how far back the common ancestor might be. In a case like that, it may well be that it’s beyond the time period — five generations or so — where the autosomal testing works best. So even if you never get a double match in both types of DNA, doing both tests is still the best way to go.

On your second question, it’s not at all unusual to find that the paper trail suggests the common ancestry between two matches is further back than the relationship range suggested. One thing that all the autosomal testing companies stress (or, at least, they should stress) is that autosomal testing can help prove you are related to someone else but it can’t usually prove how closely you’re related. Here again, the problem is the way autosomal DNA changes and recombines generation to generation. There’s a terrific short animation on the Sorenson Molecular Genealogy Foundation website — the link is here — that really explains recombination as well as anybody can.

So for this question, too, there are a couple of things to keep in mind. First, the way that these chunks of DNA are passed down is random and it may just be dumb blind luck that you and some distant cousins all got one particularly significant chunk. I see this in my own family results. My first cousin and I both match our uncle, and all three of us have matches we all share, but both my cousin and I have matches in common with our uncle that the other one doesn’t share. Pure dumb luck.

Second, you have to keep in mind that you and those distant cousins may be cousins in more than one line. It was really common for cousins to marry cousins back when the only people close enough to court were cousins! And the more lines you share with a cousin, the more common DNA you’re likely to share so that it’s easy to end up looking like the relationship is a close one when it really means you’re double (or triple) cousins. The concept is called endogamy and it really plays havoc with autosomal DNA results, particularly for populations that were kept in ghettos or whose marriages were limited by law like the Ashkenazi Jewish population. (You can read more about endogamy here.)

And, again, when you see this sort of result, it may be giving you a clue. It’s always worth it, when the paper trail says distant cousin and the DNA suggests a closer relationship, to look for the possibility that you’re related in more than one line.

Sigh… clues, clues, clues. Sometimes I think it’d be awfully nice if our ancestors’ names and lineages were written in the DNA and we could just print them out! Then again… that wouldn’t be nearly as much fun, would it? Good hunting!

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