When is a DNA segment match a real match? IBD or IBS or IBC?

The current technology for personal genome testing cannot tell you which of the two chromosomes, maternal or paternal, in a pair that an allele comes from. It can tell you that there is an AG at a specific position and a CT at the next position but not whether the A came from your mother or your father. This leads to much confusion about DNA segment matching.


Kitty and Shipley; siblings sharing 47% of their DNA

The matches that these testing companies find are for stretches of DNA that are half identical regions (HIRs). This is due to the fact that a relative who shares a DNA segment from a common ancestor with you will match you along the chromosome you got from the parent who is descended from that ancestor. Thus your new relative will match you for half the alleles in those positions. Only a sibling will share fully identical regions of DNA. Click here for a page that has a picture of the DNA I share with my brother Shipley.

For example, if my Dad gave me AAAAAAAAAAA and my Mom gave me CCCCCCCCCCC then I would seem to match absolutely everyone on that segment because every position has both an A and a C. So an ACACCAACCAC or a CCAACCCACA looks like a match, but only those with an  AAAAAAAAAAA or a  CCCCCCCCCC would be real matches. This is simplistic and the segment runs used for matching are much longer than this to try to avoid that sort of false matching. Also note than when your testing company shows an AC it is really an AT and a CG but just one of the known pairing is shown for brevity.

The term for a real match is IBD, which is an abbreviation for Identical By Descent. The term IBS means Identical by State which would apply to any false match. So in our example, the CCAACCCACAA match would be considered IBS.

However IBS is used interchangeably for false matches and matches that might be IBD.  In other words, this term is used for all matches not proven to be IBD. So IBS is also used for matches that are from so long ago that we are unlikely to find the ancestor. I find this confusing. We had a discussion in one of the facebook groups about genetic genealogy where we came up with a new term, IBC, for identical by chance, to be used for matches that are known not to be real. In other words matches that are proven to be a mix of alleles from the chromosomes from each parent.

You may ask how a match can be proven to be IBC. Well if both your parents are tested, you can prove this by seeing that you have a match that neither parent has. If all three kits are uploaded to GEDmatch you can look at this in more detail by lowering thresholds.

Another way to prove an IBC match is when you have a number of people who match you on a segment but none of them match each other there. Then it is a false match like our example above. You can compare people to each other at GEDmatch or at 23andme if you are sharing with them. At family tree DNA you have to get one of them to check the match with the other in order to see if they match each other at a specific location. The ICW function cannot tell you where they match.

Third cousin matches

My known third cousin triangulating with other family members

When we have a DNA segment that matches another person we cannot be sure that it is a real match unless it is also a match to a third person who matches both of us at that spot. This is called triangulation. Having a parent or child tested can also help a great deal with finding IBD matches. When my Dad and I or my Dad and my brother have the same match at the same location, I can be confident it is IBD since it is phased. Parent-child phasing occurs when my Dad and I both match a third person at the same spot. I know that for us it is a true match since they are matching DNA I got from my Dad. Often the match will be a little smaller when passed along to us or larger due to fuzzy boundaries.

When only Dad has a match, I try comparing to various cousins to see if they have that match too, in order to confirm that it is IBD. I have found that there are certain locations where he has many IBC matches. Comparing notes to others, I find that these spots vary from person to person.

Additional things to be aware of in segment matching. DNA tends to stay together in chunks, thus the larger the segment match, the more recent the common ancestor (CA) may be. More than one matching segment of at least 8cM is usually indicative of a close relative (4th cousin or better) unless there is more than one common ancestor or you are both from an endogamous group like Mennonites, Polynesians, or Ashkenazi Jews.

John Walden has done some statistical analysis of the sizes of segments that are likely to be IBD versus IBC, summarized at this link on the ISOGG wiki. His results show that in most population groups, segments of 10cM or larger are almost always IBD, 8-10cM segments are good about half the time, 6-8cM more likely IBC, and smaller segments more and more likely to be IBC.

So I hope this clarifies DNA segment matching and the terms IBD and IBS and I really hope some of you start to use the term IBC as well.

26 thoughts on “When is a DNA segment match a real match? IBD or IBS or IBC?

Click here to add your thoughts at the end of the comments
  1. It’s confusing enough calling them IBD and IBS and a third term IBC which sounds very much the same only compounds this. We might even say that the three “IB” are IBS with one another.

    Since, as you acknowledge, the IBS (without the IBC component removed) is a legacy from pre-genealogical time but still from out ancestors, I prefer to call these “splinters.” Which is what they are – small bits left over from larger, more useful segments.

    • What I am looking for is a good term for a completely false match, where bits from each parent appear to make a match but non-triangulation proves that they are not even splinters. That is what Identical By Chance, aka IBC, was designed to indicate … perhaps I will have to stick to false match

  2. Thanks for this! And everytime when I tried to explain what IBS was, I’d use the term “just by chance”, and I remember awhile after that when that discussion in the ISOGG FB page these other terms were brought up, as well as I was told how in my situation, there is both IBD & IBS, which is what made it confusing.

    I do like IBC though.

  3. To me, there are only two kinds of “shared” segments (HIRs) that are reported by algorithms:
    1. Those with ACGTs that match an ancestor – they match (are HIR) because they came from an ancestor – IBD.
    2. Those that are made up as various maternal and paternal ACGTs – they only match because an algorithm assembled them – you could call them IBC.

    For genealogy, these are the primary options.

    There may be shared segments from distant ancestors. How distant? What is the cutoff? Even with a distant group of segments forming a Triangulated Group, this TG is part of your chromosome map. You have DNA at that location. One parent had DNA at that location. One grandparent did, too. Probably some cousin, but maybe not. But it’s on your map. And some Matches share that segment. I think it is very confusing to try to invent a new category – a new term – for such segments. It’s all a matter of degree, which is very hard to define. Particularly when some closer ancestor covers that location.

    Personally I like the term NIBD, or NBD: Not By Descent using 3 letters.

    No matter what we call it, we have the additional issue of shared segments that have sub segments (which actually every IBD segment does). An aunt or uncle or first cousin shares some large segments with us. These large segments are made up of smaller segments, until you get down to your threshold….

  4. Love that you wrote this. I occasionally get requests from folks wanting help tracking down a common ancestor where our match is under 4 cM. It’s hard to explain to them why I’m not interested in working on such small matches.

  5. Jim I like NBD too, that is a nice one. I really want a term that will make it clear that there is no match at all, just random bits from each parent being matched up incorrectly. Many people use the term IBS for segments that are actually IBD but just too far back to find so I was looking for a new term that was less confusing.

    Thanks Kalani – notice that I mentioned Polynesians 🙂
    And thanks Jeri, glad to help out

    Thank you for the compliment Israel. The photo is from 2000 and I like that you can see the similarity between us which is why I used it 🙂

  6. I liked your IBC until Jim said NBD – that really says it. However, since I have neither parent’s results, I will have to play with my sister’s and her two sons to see what I can find.

    The other K

  7. I am searching for matches for unknown father. I’ve tested myself, children, and many cousins. Is there any significance in that of 1500 matches on gedmatch my son only has one x match -and a significant match (28Cm) at that. This x match also triangulates with me – completely- and with my daughter but with a much smaller segment. We also have triangulated matches on other chromosomes with this cousin and this cousin match does not match any of my other cousins . Could this x match signify the x that I received from my father (from his mother) and passed on to my children?

  8. Deborah –
    If this new DNA cousin does not match any of your maternal side cousins then they could well be from your unknown Dad’s side and yes that X could be from him.
    Have you checked out the methodology at the DNAadoption site? They also have a yahoo mailing list and there is one for unknown fathers as well.

  9. Hi-
    I have a confirmed through paper trail 7th cousin 1x removed and we share about 9 cM. Is this IBD because we have a confirmed relationship?

  10. Want a Laugh? IBS, IBD, IBC
    I saw the title and wondered if I might find my wife some help with her IBS (Irritable Bowel Syndrome). Thought I might be able to find out whether to blame her father or her mother.

    Glad I read further! LOL All these initials are used for the the bowel as well: IBS (above); IBD – Irritable Bowel Disease; IBC – Irritable Bowel (Constipation).
    Anyway. A good laugh.

  11. Right now the main stream of thought is that 7cM’s or possibly as low as 5cM matches should be used to confirm IBD cousins. The math for triangulations hasn’t been developed as far as I can tell. Is this true? They simply know that triangulations are a significant means of confirming ancestry that also has historical documentation.

    I believe that a brilliant mathematician will be able to improve our statistical analysis of IBD’s vs. IBS’s via the following patterns.

    1. Ancestry has stated that mathematically out of 30 cousins found descending from the same MCRA, there is only a 4% chance of finding a triangulation between them. (Therefore, statistically shouldn’t each triangulation increase the likelihood that any cM’s in common between 3 cousins descended from the same ancestor verify IBD’s vs. IBS’s?)

    2. If someone did a study on how many times even persons with the same historical ancestors in common have any cM’s in common, shouldn’t that too refine the chances that IBS’s are actually IBD’s?

    3. From Blaine Bettinger’s chart that many of us have seen, the following close relationships can have as little as 0 cM’s in common: 2C1R, 2C2R, 3C, 3C1R, 3C2R. Again, is that a factor that might be able to be used to refine into this analysis as compared to any cM’s in common back in time?

    Obviously, we are still in the infancy of this science. I have great hopes that the future will help us do improved analyses to distinguish between IBD’s and IBS/IBC’s. What do you think?

  12. I appreciate your desire to simplify, to substitute more intuitive terms, and I do see IBC used a little. NBD is a simpler idea, but still a new term too. I think the simplest though, and therefore the most likely to catch on, is to drop all terms but IBD, and make the question ‘IBD’ or ‘not IBD’ (to almost quote Hamlet).

    Your article is well written, a good persuasion for the use of IBC. And if it sticks, I’ll use IBC. But I do feel that ‘not IBD’ is simpler, one less term to remember.

    • Rob, frankly in my talks I use the term false matches and try to avoid IBD or any of the others. Too much jargon. Your suggestion of not IBD is just fine.

  13. If there are two related cousins that share segments separately with a thrid person, one is above 20cM the other falls into the <5cM category on each segment, and the three only share one segment together, how does that show os IBD or IBS? I am trying to find my grandson's biological family. One cousin is considered a first to third cousin and the other is considered as a 2-4.

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