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I've done autosomal DNA tests on ftdna for myself, my mother and paternal grandmother. I've transferred the results to MyHeritage, where I have a match with a person X with the largest segment of 27cM. Neither my mother nor grandmother match with X, so I assume it has to be a match from my paternal grandfather whom I have not tested. Looking at the match’s ancestory I can indeed identify common ancestors with my paternal grandfather. However, looking at the matching segment's location on the chromosome map, I see that it falls in an area where I also have a match with my paternal grandmother, which means that I cannot at the same time also have segments from my paternal grandfather in that same location. There's a clear contradiction there. Should I regard it as false positive or is some other explanation possible?

Edit: the following image is a screen capture from MyHeritage's chromosome browser. In red are my matching segments with X (6,3cM in cr 18, 7,9cM in cr 20 and 27,2cM in cr 22), in orange my matching segments with my paternal grandmother. As said, X and my paternal grandmother do not triangulate with each other. As seen, all of these three chromosomes (18, 20 and 22) are entirely from my paternal grandmother (the other pair of the same chromosome being from my mother), therefore there cannot also be any DNA from my paternal grandfather in the same chromosomes.

enter image description here

Update: The matching segment in chromosome 22 has a triangulation group consisting of possibly 10+ persons, including me, my paternal grandmother and X. Everyone triangulates with everyone, except my paternal grandmother and X, who both triangulate everyone else in the group, but not each other.

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  • Sorry that I'm not familiar with how ftdna presents the location of regions. I'm confused by this part of your question: "tested ... my mother and paternal grandmother ... largest segment of 27cM that does not match either my mother or grandmother ... the segment falls in a region where I already match with my paternal grandmother" It doesn't make sense that ftdna is reporting you have a large segment not matching your paternal grandmother and that this segment is in a region that does not match your grandmother. Perhaps there is some confusion as to what ftdna means by region?
    – Castedo
    Oct 4, 2021 at 12:19
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    Nov 9, 2021 at 2:37

2 Answers 2

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This question is a bit confusing. When you say "Neither my mother nor grandmother match with X", I'm assuming you mean your paternal grandmother.

You said "looking at the matching segment's location on the chromosome map, I see that it falls in an area where I also have a match with my paternal grandmother". You also said that your match with X is only 27 cM. The matches between you and X, and you and your grandmother, do not have to be exclusionary. In other words, your conclusion "means that I cannot at the same time also have segments from my paternal grandfather in that same location" is incorrect.

Possiblity 1) Your paternal grandparents are (perhaps distantly) related to each other, and share DNA in that position. You said that your grandmother does not match, but what we cannot see is whether there is zero match at all, or just a smaller match, between her and X at that position. A bad read anywhere within that segment in her test could have artificially prevented the segment from matching.

Possiblity 2) You're a bit unclear as to the "area" of the match. "it falls in an area where I also have a match" - is this talking about a) that entire chromosome, b) some part of it, or c) just the specific matching segment?

  • a) the issue is comprehension; namely, you inherit bits of each chromosome from each parent. So even if your paternal grandparents are entirely unrelated, you could have bits from each of them within the same chromosome.
  • b) similar to a, just on a smaller scale.
  • c) that leaves only the possibility that you're saying that the entire specific segment matching between you and X is one in which you separately have a match with your paternal grandmother. However, that statement cannot be true without also saying that X and your grandmother have a match in the same position (unless possibility 1 above). If you and X have a match, then for that segment, you and X are interchangible. Logically, you cannot then have a match in the exact same spot with your paternal grandmother, and yet X not also have a match with her in that same position.

As for a false match, a 27 cM match is outside of any real possibility. In theory, sure, there might be some infinitesimal chance that you get exactly the wrong reads in enough places that a false positive of that size is created. That is so massively unlikely, however, as to be essentially impossible. A false positive would be expected to max out around 6 or 7 cM; a match of 27 cM is essentially certain to be a real match.

Let me just follow up on that last a little; just because it is a real match, it does not mean that it will be easy to identify the relationship between you and X. I have several people with matches in the 25 to 38 cM range that I've been working with for multiple years, where we each think we know roughly where the match between us should be in our respective trees, yet we've not yet found the connection. If this is the only sizeable matching segment between you and X, the connection could easily be 5C3R, 6C1R, 6C2R, 7C, 7C1R, 8C, and would require you both to have quite deep trees in exactly the right places to identify the common ancestor.

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  • I updated my original question with an image. I hope this makes it easier to understand. As seen from the image, the chromosomes 18, 20 and 22 where I have matching segments with X are entirely from my paternal grandmother. My paternal grandparents come from quite a different geographical areas, making it very unlikely that they're related (though not entirely impossible, but still unlikely). As far as I can tell, there are only two possibilities in such situation, these segments are either a) false positives, or b) false negatives for my mother or grandmother.
    – CuriousM
    Nov 8, 2021 at 21:56
  • I think you may either perhaps be misunderstanding the result from matching with your paternal grandmother, or there is something at issue with the match being performed. You do not inherit entire chromosomes from a parent, let alone a grandparent. Rather. you inherit pieces of each chromosome. The chances that you inherited not one but multiple chromosomes in their entirety from a grandparent are essentially zero. Take a look at genetics.thetech.org/ask/ask445 ; it has some useful images to conceptualize this.
    – BrianFreud
    Nov 9, 2021 at 17:38
  • No, I understand fairly well how inheritance works, and no, you don't inherit pieces of chromosomes from your parents, you inherit 23 full chromosomes from each of your parent, which are then recombined between the autosomes of the same chromosome. Non-recombinant inheritance is possible and does happen. I don't know how common or uncommon it is, but it is possible to have an exact copy of one of your parents chromosomes. See this guide for example: smithplanet.com/stuff/gedmatch.htm Anyway, this is not exactly the topic of my question.
    – CuriousM
    Nov 9, 2021 at 19:15
  • Right, it can happen that you manage to inherit a chromosome in its entirety from one parent, rather than a recombinant mix from both. However, the scenario you present is one in which both you and your father would have had to have that unlikely event happen not once, but three times. While possible, the chances of that being the case are very very small. I'm assuming however that the orange bars represent your own DNA results. Are you saying that they are your paternal grandmother's results instead?
    – BrianFreud
    Nov 10, 2021 at 1:37
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    Not directly related to the original question, but as it has been under discussion in these comments, there's an excellent article explaining DNA crossovers and their frequency: dna-explained.com/2019/09/17/… Citing the article: "In total, 94% of offspring will inherit between 4 and 12 chromosomes containing DNA exclusively from a single grandparent. In the 100,000 simulations the child always inherited at least one chromosome without recombination."
    – CuriousM
    Nov 11, 2021 at 12:17
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I'm unfamiliar with ftdna but consider that we are diplod organisms meaning you have two sets of each autosomal chromosome.

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