DNA match says my husband and I are 5-8 cousins...can you help me figure out which branch needs more research

+5 votes
401 views
This is what I get when I put in our two test results in a one-to-one comparison.

Chr 17

Start location 69,854

End Location 3,386,204

Centimorgans 10.9

Largest segment = 10.9 cM

Total of segments > 7 cM=10.9 cM

1 matching segments

Estimated number od generations to MRCA = 5.2

Is this enough info to figure anything out?

Thanks in advance!
in Genealogy Help by Pam Smith G2G6 Mach 1 (10.6k points)
retagged by Lynda Crackett

5 Answers

+10 votes

It might give you some clues if you look at other matches who share the same segment. 

You can also get an indication of potential relationships by putting the number of cM into the shared cM tool on DNA Painter.

by Lynda Crackett G2G6 Pilot (630k points)
Dumb question here - sorry this is all new to me....how do I know if they share the same segment? I am on GEDmatch...how do I look that up on my list?

Shared cM tool is pretty vague - we know the connection is distant, but want to figure out which line to search in....
+10 votes
Pamela: This result says that you share exactly one segment of DNA with your husband. It is on Chromosome 17 starting at position 69,854 and going to position 3,386,204.

Without more information there is no way to know how you are related (DNA doesn't tell us where it came from).

It seems like the best way to track down how you are related is if you and your husband's family are willing to do DNA tests and share with you. For example, if your Mom also shares this same segment with your Husband's Dad, then you have narrowed it down (to your maternal half of your tree and his paternal half). If your Mom doesn't share this DNA with your husband, then it's probably through your Dad's side. If you can get many relatives to share DNA, you will be able to narrow your search down a lot!

If neither of you have family who are willing to do DNA testing (or you'd just rather do some research before asking them all) you can try to find other people on GEDmatch using the one-to-many match feature and see if any of them share the same segment you have (have a match on Chromosome 17 which starts at or before where yours starts and goes to or after where it ends). If so, then all three of you probably share a common ancestor and that might help you figure it out.

Also, note that your 10.9 cM is pretty small match. Most websites won't even show you matches smaller than 7 cM. I don't know the probability, but it's possible that a short match like this could be spurious (not actually indicative of a recent ancestor), so be prepared for that possibility.
by Shawn Ligocki G2G6 Mach 1 (16.6k points)
+8 votes
Like Shawn says, its probably just as likely that its just random, and that you don't actually share a common ancestor (not close enough to find, anyway)

But one thing you can try, is to look for shared matches. Look at your matches and then his matches, and see if any of them are in common. That might give you some clues as to where to look next.
by Dennis Wheeler G2G6 Pilot (537k points)
+6 votes
On GEDmatch use the "People who match one or both of 2 kits" option, and on the search page enter the kit numbers for each of you.

Then you will need to go through each resulting mutual match in the first table & check that it is an actual match on the same chromosome / segment.
by Vivian Egan G2G6 Mach 7 (75.4k points)
+5 votes

yes for Shawn, Randy, and others. (Yeah; the new emojis might get a little annoying in the wrong hands. Ahem.) This will be on the geeky side, Pamela, so maybe just ignore me and come back later if you decide to get deeper into the genetic weeds.

The "are your parents related" tool at GEDmatch that Randy mentioned is always a good first stop for anyone who's uploaded data to GEDmatch. What it checks for are what's called runs of homozygosity. Put really simply, all our bits of DNA come in one of four flavors, or letters (strictly, nucleic acids, but who's counting): A, C, G, and T. If you open up your raw data file in a text editor, you'll see two columns in each row (a row being a tested base pair, or SNP), and each of those columns contains one of the four letters. If the two letters in a given row are the same (e.g., AA, CC) then they're called homozygous, because they have the same pair of letters at that position. If you have a whole boatload in a contiguous sequence where two letters match each other at a given position (a locus), you have a "run."  smiley

GEDmatch doesn't have any extra options you can set for this utility; it's as simple as enter a kit number and go! And generally speaking, it's just fine as it is. If the two parents had fully identical DNA for long stretches, it will show up and indicate that the parents may share recent ancestors. If enough time (as in number of birth events) has transpired since those common ancestors, the tool won't indicate the parents were related.

It's important to know not just for endagomous populations, but also for families that might have relatively recent (or multiple instances of) pedigree collapse. This affects the average amounts of DNA sharing expected, and is why I always recommend it as the first stop for a new kit. For those who want to (or may need to, to get more information), the GEDmatch utility uses a technique developed by David Pike of Newfoundland. He has some free utilities available that allow you to get more granular, for example to set how many base pairs in a row must be identical to be considered a "run," and how many non-identical pairs are allowed before a run is considered broken and not really one run.

As others have noted, that 10.9cM segment isn't very big and, though it isn't within a known population pile-up region (segments along chromosomes that tend to be shared by very broad segments of a population because they aren't genealogically pertinent; they're passed down over many hundreds or even thousands of years), it is near the very beginning of chromosome 17 and is in an area that contains a number of protein-encoding genes...90 of them, in fact. In addition to broad population pile-up regions, we all have our own haplotype--or familial--pile-ups. Here's a recent article by Debbie Kennnett that explains it very well and includes some nifty visuals. Areas within protein-encoding genes--at least a lot of the important ones--don't mutate all that much. Understandably: rolling the mutation dice for important genes is risky. One example is an vital area of what's called the HLA complex on chromosome 6; it's involved in your autoimmune system. This particular area lives roughly between base pairs 25Mb (mega-base-pair notation) and 35Mb on Chr6. Have a look at the graph Debbie posted of her chromosome 6 and the frequency of matches as downloaded from her kit at Family Tree DNA. Can you spot the HLA region? Betcha did.  wink

The point here being that you and your husband might have a ton of other matches in and around that segment on Chr17. There's a GEDmatch Tier 1 tool that let's you directly compare segments among all your matches, but as Vivian noted, you can accomplish mostly the same thing with the free "people who match one or both of 2 kits" option.

Last geeky wrench that I'll throw in the machinery: We tend to think of the segment sizes we see from GEDmatch and the testing companies as being a precise measurement. They aren't. Confusion rises especially when some reports show centiMorgan values down to the 1/100th. The centiMorgan ain't even a physical measurement at all; it's a linear equation interpolation of Base Pair A to Base Pair B, estimating the number of likely DNA crossover occurrences and using that to arrive at a guess about the relative genetic distance. <cough, cough> But there's a massive amount of mathematical estimates, probabilities, imputations, genotyping and, yes, educated guesswork that goes into the DNA results and predictions that we see. Even the start and stop base pair numbers you see from GEDmatch aren't precise (though they look like they are) because all that's tested are about 630,000 to 700,000 identified base pairs (SNPs) among the roughly 3.2 billion that make up your whole genome...or about 0.023%. So the test results can only look at the first and last tested base pair in a sequence, not the actual physical base pair numbers. Top it off, we call a segment a segment because we see, give or take a margin of error (ya know that bunching mismatch option in GEDmatch?), a sequence of identical SNPs...but there may be millions of base pairs within that sequence and we're only assuming that they're all the same because the thousand of them we actually tested were identical.

For the most part, it all works well enough for our genealogical purposes. But it pays to be informed, and to understand that segment sizes are not always what we assume them to be. (I'm flashing back to Diego Montoya in The Princess Bride: "I do not think that word means what you think it means.")

Another case in point is that what we have reported to us in cMs for segment size are what's called "sex-averaged" values. Not only do different versions of the human genome maps differ slightly from one another (and GEDmatch uses hg36 while almost all academic studies now use an iterative-level of hg38), but the male and female genomes differ significantly when it comes to the linear equations that estimate centiMorgans. The female genome experiences 30% or more crossovers than the male.

Let's take a look, using hg36 which is what GEDmatch uses, at that segment of yours on chromosome 17. Your computed value for it is 3.1cM; for your husband it's 17.7cM...ergo the averaged value you see of 10.9cM. When dealing with small sex-averaged segments, the actual gender-mapped value can completely zero out what was thought to be a segment of sufficient size. Once you're into the area of around 15cM, you're generally safe that a segment really is a segment. Randy talked about general reliability of small segments, so I won't other than to note an informal study done by a knowledgeable guy named Tim Janzen. He found that 7cM segments survived traditional trio phasing and proved to be valid only 42% of the time; 11cM segments proved valid 90% of the time; and 15cM segments 100% of the time.

I told you not to read this, and just to bookmark it and come back if you decide to really dig into genetic genealogy. You certainly may want to investigate that 10.9cM. I would, for fun if nothing else. But if there is pedigree collapse causing it, it's likely at least at the 4g-grandparent level; almost certainly not closer than 3g-grandparents. So it won't become readily apparent exactly which branch it's on without some significant digging. A popular, free app that I've come to love that can help with the effort is called DNA Painter. Other apps can do the same things, but DNA Painter is easy to use, flexible, you can share your results with others and, of course, free. The big plus is that using a chromosome painting tool like this to chase down that 10.9cM segment will mean you'll also be filling in matching segments for a bunch of cousins. The result is that you'll have most of the homework done when you discover the next new cousin.

by Edison Williams G2G6 Pilot (309k points)
There's so much great info here, and it's going to take me awhile to digest it! But I want to thank you for taking the time and trouble to write, and for your expertise. I can't wait to get my teeth into this problem...and hope to figure it out!!

OK, so I was just going through all my "People who match one or both of 2 kits" people for a new fourth cousin (4C) that showed up matching me on GEDmatch. The last one got a little weird.

The thing is, 4C & I both match this last person on the list, and match them at the same place, on the same Chromosome. That's all great, but we don't match EACH OTHER there!

So I guess I match the dad's side, and 4C matches the mom's side, or vice versa, and they both just happen to be on roughly the same 10cM segment? That sounds kind of crazy!

I don't know if it's relevant, but while I said he's a 4C, he's also a 5C. It's crazy, because my gt-gt grandparents were 2nd cousins, and HIS gt-gt grandparents were (1) a sister of my gt-gt grandpa and (2) a 1st cousin of my gt-gt grandma, on the side of the family the brides & grooms have in common. So they were 2nd cousins, too, both gt-grandchildren of the same guy that my gt-gt grandparents were also the gt-grandchildren of. Basically, 3 pairs of our 4gt grandparents are the same (not just 2, like regular 4Cs), and 2 of those 4gt grandmothers are sisters. We both have the same 5gt grandfather (& wife) on our pedigrees in two places. Whew!

I'm trying to imagine the circumstances that would cause this DNA-matching puzzle, but to quote Montoya again "Is too much!" (I don't think his first name was "Diego", BTW. I don't know how to spell it, but it sounds like Inego.)

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