Parent-Child Non-Matching Autosomal DNA Segments

Recently, I had the opportunity to compare 2 children’s autosomal DNA against both of their parents.  Since children obtain 50% of their DNA from each parent (except for the X chromosome in males), it stands to reason that all valid autosomal matches to these children not only will, but must match one parent or the other.  If not, then the match is not valid – in other words – it’s an identical match by chance.

If you remember, the definition of a match by chance, or IBC (identical by chance) is when someone matches a child but doesn’t match either parent.

This means that the DNA segments, or alleles, just happen to line up so that it reads as a match for the child, by zigzagging back and forth between the DNA of both parents, but it really isn’t a valid genealogical match.

You can read about how this works in my article, How Phasing Works and Determining IBD Versus IBS Matches and also in the article, One Chromosome, Two Sides, No Zipper.

The absolute best way to determine if a match is a valid match or not, valid meaning that the DNA was handed down by ancestors, not a match by chance, is to compare a child’s matches against both parents.  By doing that, we can quickly identify and isolate matches that aren’t real.


In the example above, you can see that Mom contributed all As to me and Dad contributed all Cs to me.  Joe has alternating As and Cs, so he is a match to me on every location.  However, he only matches my parents on half of their locations, so he is not a match to them, because it’s only chance that caused him to match me on those allele values in that order.

DNA matching programs have to take into consideration both allele values in their match routines, since you carry a value from your mother (A above) and a value from your father (C above), and they are not labeled as to which parent they come from.

Valid matches will also match one parent or the other.  After all, the child received all of their DNA from one parent or the other, so for someone to be a valid genealogical match a child, they must match a parent.

Some time back, when I was matching to my own mother’s DNA, I noticed that I matched her on about 40% of my matches, which left 60% to either be matches to my father or identical by chance.

Notice, I’m not talking about IBS, or identical by state, because that phrase is used to mean both identical by chance and identical by population.  Identical by population means that you did in fact inherit the DNA from an ancestor, but it’s either too far back in time to determine which ancestor, or that segment was present in a specific, probably endogamous population, and you could have inherited it from any number of ancestors.

So, identical by population is identical by descent, but we just can’t tell who we got received that DNA from.

  • IBC – identical by chance – not a valid match – you happen to match someone else on a particular segment, but it’s because the match software is jumping back and forth from your mother’s side to your father’s side.
  • IBD – Identical by descent – you share a common segment of DNA because you and another person(s) inherited that DNA segment from a common ancestor who you can identify
  • IBS – Identical by state – currently used to be both IBC and IBS, where IBS means that you did inherit this DNA from a common ancestor, but it’s so far back you can’t determine who, or that segment is so common within a particular population you could have inherited it from a number of people.

Now a 60-40 parental split is certainly possible, especially if one parent was from an endogamous population, which would mean more matches, or one parent was more recently immigrated from the old country, which would mean fewer matches.

However, without my father’s DNA, which is not available, we’ll never know.

Since that time, I have obtained access to 2 sets of child plus both parents DNA results, so I wanted to take a look at how IBD versus IBC stacked up.  These comparisons were done at Family Tree DNA.

Total Matches Non-Matching Either Parent Percent Non-Matching
Child 1 959 133 13.9
Child 2 1037 133 12.8

Based on other evidence I’ve seen, this percentage seems about right, but the amount of shared DNA and the largest segment size surprised me.  Keep in mind that the smallest possible segment size is 7cM which is Family Tree DNA’s lowest single segment threshold to be counted as a match (assuming you meet the 20cM total threshold first.)  If you match, they show you your matching DNA down to 1cM, but these tables are measurements by the 7cM matching criteria only.

In plain English, this means that in this case, 12% and 13% of these matches were identical by chance, or false matches.  These matches included people who shared up to 57cM of data and the largest block was 15cM.

Largest Shared cM Largest Longest Block
Child 1 46.87 14.38
Child 2 57.06 15.18

Could something else be causing this?  Certainly.  Some of these non-matches could be read errors in the files.  I’d certainly want to take a look at that if any of these became critical.  Another possibility could be that valid match segments are “stitched together” by IBC segments creating longer segments in the child.

An alternative to check validity would be to download the files to GedMatch and see if the pattern continues using the same match criteria.  Of course, testing at multiple labs and downloading the results to compare at GedMatch likely removes the issue of read errors in the first set of files.  And if you really, REALLY, want to know, you can look at the raw data files themselves.

Just so you know, this wasn’t an anomaly with just one high read.  Here are the highest 25 entries from Child 2, or about one fifth of her total mismatches.  Only a few were in the 3-5th cousin range.  None were closer.  Most were 4th or 5th to remote.

non-parent matching relationship range

If you want to do these comparisons yourself, they are easy to do if you have a child and both parents who have tested at Family Tree DNA.

On your Family Finder matches page, at the bottom, in the right corner, there is a button to download matches.

download button

I download the matches into separate spreadsheets for the child, mother and father.  I then color all of the rows pink in the mother’s results, and blue in the father’s results, then copy all three to a common spreadsheet.  You can then sort on the match name and this is what you’ll see.

non-match example

What you’re looking for is white (child) rows that don’t match either a blue row (father) or a pink row (mother.)  Don’t worry about pink or blue rows that don’t have matches. It’s normal for the DNA not to be passed to the child part of the time, so these are expected.

In this example, all white rows matched one parent or the other, except for Winnie Whines.  I colored this row red and added the Comment column where I entered the number of this non-matching entry.  When I’m finished comparing and coloring, then all I have to do is sort that column, bringing all of the nonmatching rows together.  I copied those nonmatching entries into a separate sheet so I could sort those alone and obtained the largest shared and longest segments.  To determine the percent, just divide the total number of nonmatches, in this case, 133, by the child’s total number of matches, in this case, 959, giving a non-parent-match percentage of 13.9%.

So, the take-home message is that not all small segment matches are genealogically irrelevant and not all larger segment matches are genealogically relevant.  Thank goodness we have tools and processes to begin to tell the difference.

So, if you don’t have both parents to compare to, and you’re wondering why you just can’t find a common ancestor with someone you match, the answer might be that they fall into your 12 or 13% that are IBC matches.

If you perform this little exercise, comparing a child to both parents, please feel free to post your results in the comments section along with any commentary about endogamous populations or special circumstances.  It really doesn’t take long, probably about an hour total, and the results are really interesting.  Plus, you’ll have eliminated all those irrelevant matches.

I’ll be writing more about this interesting experiment in coming days.

47 thoughts on “Parent-Child Non-Matching Autosomal DNA Segments

  1. Roberta–EXCELLENT write up! I only this past year recognized the value of this myself–we are all still learning and for some reason that ‘bulb’ didn’t go off sooner in my head! Last month I ordered kits for all of my siblings, hoping to validate the majority. I’m wondering has anyone come up with an average for how much of a total for a parent you can cover validating IBD vs IBS/IBC for a parent with a given number of children tested for comparison? I’m assuming the more children the better your chances of getting close to 100% coverage! I suppose next month when I have results from all these kits I’ll have an idea, but curious if anyone has looked at that already?

  2. Unfortunately I can’t perform this test as I don’t have parents and children to compare but I certainly do seem to have a lot of matches that I can’t explain. In particular I have a number of matches, both male and female, that show large segment X matches (20+ cM) but little else. Is that in itself and indication of an IBC match on X? I recently noticed that FTDNA does not include X matches when reporting the largest segment. Is this why? I only noticed these large X segments when I downloaded the full segment table. I am an Ashkenazi Jew with over 4200 FTDNA matches.

  3. I have sets of parent child data (same grouping) at all three companies.

    For FTDNA my IBC matches were 20%
    For 23andMe, my IBC matches were 13%
    For Ancestry using their current algorithm they are 34%, and the V1 matches were 51%. Interestingly I currently have 3 of 27 (so about 10%) 4th-6th cousin matches appearing on my list, but they do not appear on either of my parents lists. According to Ancestry, those are “very high” confidence matches.

    • 34% (false positives in the child or false negatives in the parent) at AncestryDNA is disconcerting. I wonder whether that represents a problem with the phasing methodology (phase switch errors or whatever), or whether the parents have pile-ups that are within acceptable limits in the child, or ??? It does appear from your 4th-6th cousin data that false negatives in the parent account for some percentage of the discrepancies.

  4. Would be interesting to do this experiment in three generations. See how many of a grandchilds matches that doesn’t match either grandparent. I guess it would be even larger than 12-13%

  5. This is interesting. I’d like to do a similar comparison. However, my husband, my two kids and I have all been tested via AncestryDNA, not ftDNA. How can I do a similar test with our data from AncestryDNA?

    • You can’t get the segment sizes, but you can make a list of all the matches for each person and then compare them. Unfortunately, I don’t know of a way to download your matches from Ancestry, so it would be a manual process.

      You can also download all 3 individuals results to (when they come back up from maintenance) and do a comparison there. You can copy and paste results into a spreadsheet at gedmatch, so it would be easier.

      You can also transfer all 3 sets of results to Family Tree DNA for $39 each which lets you play in that match pool as well and with tools you don’t have at Ancestry.

      • To download your matches, go to your settings page. You can download all your V1 matches there. This is also where one would find the link to download one’s raw data also. If I remember correctly, the last time I downloaded my matches, it also had my most recent ones included. Give it a try.


      • Unless your parents also tested before the great phasing purge, it won’t be apples and apples. I’m not positive your current matches are all in there either – but that you can check.

      • Roberta,

        You are correct, recent matches are not listed. I just checked. Parents testing for me will be impossible. The best I have is an aunt at that tested at FamilyTreeDNA.

        Thanks for the fantastic blog and keep up the great information. I’ve learned so much.


      • Thank you for the information. I did lose some of my known matches, confirmed by a good paper trail for both myself and my 7th cousin in Denmark, when AncestryDNA did their change.

        I have uploaded my data to ftDNA, but had my husband tested at ftDNA. If I upload my kids ancestryDNA data to ftDNA and compare that to my uploaded ancestry data and my husband’s ftDNA data, will I be comparing 3 apples to another apple, or 3 apples to an orange?

      • If your husband took the Family Finder test at FTDNA, then you’ll be comparing apples and apples. If he took the Y or mtDNA, then just order the Family Finder for him or test him at Ancestry and upload his results to his account at FTDNA too.

  6. I have a question about this. I have tested 5 generations on my paternal side and 4 generations on my maternal side along with various cousins to specific line. I have matches that do not match my mother but match my maternal aunt, 1st cousin, etc. on the same line. Why would they match all my other maternal family and me but not my mother?

    • Perhaps a read error in your mother’s file? In situations like this, I often download results to GedMatch to check, and I also run the same people at different labs, preferably FTDNA and 23andMe.

  7. What are the SNP count for the IDC segments? What is the smallest/average/largest SNP count?

    The Largest SNP of your “Parent-Child Non-Matching Autosomal DNA Segments” might give a reasonable threshold for IBD.

    • Please keep in mind that GEDMatch is a largely FREE service providing a wealth of information that big, wealthy Ancestry refuses to do. Yes it is down for maintenance sometimes. When this happens send $$ to help them get up and running. When they are available, you will find a truly amazing amount of invaluable information.

  8. I have tested my dad and maternal grandparents. My dad tested at FTDNA and myself and my grandparents tested with Ancestry and then transferred to FTDNA. My results are:

    IBC: 11.8% (1460 Total matches, 172 not matching father or grandparents)
    Largest shared cM: 51.78 cM
    Longest Segment: 13.38 cM

  9. I did the test with my own and my parents FTDNA results, 91 of my 439 matches does not match any of my parents. That gives 20.7 % IBC.
    Two of them are classified as 4th-remote, the rest as 5th-remote. Longest block is 12.27, most shared cM is 67.40

  10. I only have one parent to match against, but I can say that some of the matches to me or my sister that do not match our father on FTDNA, do match our father when the data is brought over to Gedmatch. Therefore, a small percentage are misreads at FTDNA. I haven’t worked out the percentage, but it appears to be very small.

    It would be interesting to know if your 15 cM longest block IBC match would be smaller at Gedmatch. A few of my results from FTDNA show a significantly smaller longest block size at Gedmatch.

  11. Of my 384 matches (as of last December), excluding my mother and father, 73 (19%) are IBC. Both my mother and father are of Swedish descent and are not related within the past few hundred years (as far as I can tell). The longest segment they share is 4.9 cM (according to GEDmatch).

    3rd Cousin – 5th Cousin: 1 of 41 (2.4%)
    4th Cousin – Remote Cousin: 3 of 83 (3.6%)
    5th Cousin – Remote Cousin: 69 of 247 (27.9%)

    Longest block 16.56 cM: 1
    Longest block 13.1 cM: 1
    Longest block 11.28 cM: 1
    Longest block between 10.00 – 10.99 cM: 3 of 41 (7.3%)
    Longest block between 9.00 – 9.99 cM: 7 of 71 (9.9%)
    Longest block between 8.00 – 8.99 cM: 37 of 111 (33.3%)
    Longest block between 7.70 – 7.99 cM: 23 of 54 (42.6%)

    Shared cM 61.8 cM: 1
    Shared cM between 50.0 – 59.9 cM: 4 of 47 (8.5%)
    Shared cM between 40.0 – 49.9 cM: 14 of 105 (13.3%)
    Shared cM between 30.0 – 39.9 cM: 37 of 152 (24.3%)
    Shared cM between 20.0 – 29.9 cM: 17 of 64 (26.6%)

  12. Question:I have a high Surname Y-DNA match at FTDNA (65 of 67).He is also my only Surname Y-DNA match other than a 1st cousin (37 for 37).However when I get to Family Finder I have several Surname matches but no one matches me or the previous 2 people mentioned on Y-DNA when I run a Matrix can I make a fairly safe assumption all the Family Finder matches are likely coming from the FEMALE Surname side of my Williams line??

  13. I wonder about V3 versus V4 at 23andMe. I share an Ashkenazi segment on chromosome 18 with my Mother based on my phased ancestry composition and split view yet she doesn’t match my Ashkenazi cousins on the segment who match me in the 5 cM to 8 cM range. I guess it could be a misread SNP or missing SNP as well.

  14. Four points:
    1. After the last FTDNA Admin conference, I talked with the senior staff for several hours after the Lab tour. Their research indicated a non-IBD rate of about 5%. Their version.
    2. At GEDmatch I have found Matches which match my Dad, which didn’t match him at FTDNA.
    3. At GEDmatch I have also found several instances of two adjacent segments at one company and a full segment at another company – indicating the companies handle no-calls differently (or a parent has a no call that prevents a match).
    4. In my fairly complete chromosome map of Triangulated Groups, I have a number of cases of 7-10cM Match-segments that don’t match Dad at FTDNA, but triangulate with others in the TG. They might be IBS, but I tend to think they would match using the GEDmatch criteria.

    These all result in false negatives, which would reduce the false positive percentages. I hope to post some of these details from my spreadsheet soon.

  15. Thank you for addressing this issue that has had me scratching my head. My parents and I have tested at Family Finder and I am in the process of mapping us. One of the big surprises has been how many of my matches do not match either of them. I’ve been guessing about 10% but did the math today and it actually comes to 13.9% (131 of 943 matches).

    The thing is that about 2/3 of those 131 settle in matrix-matched clusters in my map such that I can identify them as either likely maternal or paternal. With whom they are ICW and where they match they SHOULD also match one parent or the other but don’t. They are all distant matches (4th or 5th to remote) and generally have smaller total shared cM and longest block cM numbers. My thought is they could be actual matches that barely meet the match criteria for me and just miss my parents. This might, possibly, could be explained by another oddity I found that has me equally confused. I downloaded all matches from the ftDNA chromosome browser for the three of us and put them in a spreadsheet then sorted so I could see how all my segments from common matches matched my parents. I expected to find that most of them would and was astounded to see that, while long segments from every common match were very clearly shared, a great number of the smaller segments were not. Of 8817 reported segments of any size from my matches who also match either Mom or Dad, 5866 of these segments (66.5%) do not overlap either parent. In fact not a single match had all my segments completely subordinate to a parent. I can only identify 1523 segments (17.3%) as clearly shared with Mom and 1428 (16.2%) with Dad. I don’t know the explanation(s) for all the small unshared segments, but I can see from this the mechanics of how someone who is a valid match might just match me and not a parent, especially if the match is marginal. The longest block might be over the 7cM criteria for both of us, but the total that includes the varying smaller segments may or may not make the required 20cM for the parent. Does this make any sense??

  16. This is a great post—thanks so much. I have had my mother tested, but not my father, and your post is making me think that I should. However, since it’s my mother’s family I am most interested in researching, what I really wish I had was my grandparents’ DNA. Oh, well. Thank you for this very clear and helpful explanation.

  17. I did this comparison for my daughter’s matches to my husband’s and mine; she has 18.7% non-parental matches, largest cM is 54.03, longest segment is 16.87 cM. This seems high relative to everyone else’s above considering she only has 694 matches total. Could this be because of the endogamy tangling up both my paternal and maternal lines?

  18. Loved this article! I’m lucky enough to have both my parents’ samples at FTDNA (my dad did his at Ancestry, and transferred). Also, 2 of my 3 sibs had their DNA tested at FTDNA. I have 629 matches (other than parents and sibs). 108 (17.2%) of those have no match to my parents or to either of my sibs. An additional 49 matches have no match to my parents, but they do match 1 or both sibs. Together, that is 25.0% of my matches which don’t match to a parent. (I’ll post later on the % of non-parental matches each of my sibs has. )

  19. I’ve enjoyed your articles and find them very helpful. My two sons, my husband, and I have all tested with Family Tree DNA. In going through our matches, many of my sons’ matches do not match my husband or me. I have two questions: 1) Am I correct in identifying these matches as IBC, since they do not match my husband or me?
    In addition to these four accounts, I manage 32 other relatives’ accounts, and many of my sons’ IBC non-parental matches do, in fact, match other relatives, such as 1st, 2nd, 3rd cousins, grandparents, aunts, great-aunts, uncles and great-uncles. 2) Would this imply that these matches are also IBC to the other members in my group? Thanks for your response,

    • Since you do have others to match to, some of your son’s non-matches to you and your husband may be read errors which then show up at matches to others. I would keep an open mind about some of those matches.

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  21. My wife recently tested her parents and herself at Ancestry. Out of 458 matches, she shared 32% with her mother and 29% with her father. 40% of her matches were not shared by either her mother or her father.

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  25. Thank you for this article, my mum and I have tested, and my brother and dad are about to, so your example fits us perfectly! Keen to learn more 🙂

  26. How are you. So my story is that my daughter matches all 15 tested strands of locus of that the correct term. But with me she only matches 9 out of the 15 strands tested. Hypothetically speaking i know some dna might not be passed down by me to my child…so can it be a possibility that it can be my child but shes a rare case where some of her dna that came from me was passed down from someone from my past like great grandparents?

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