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I thought I had grasped the meaning of triangulation. Until I saw this match in MyHeritage (see below).

What is happening?

It looks like the two segments that have the triangulated match are single segments, but why is the triangulated match not starting at the left of the top segment. Let's say the triangulated segment (part) is on the paternal chromosome.

Is the left part of the top segment on the maternal chromosome, or the left part of the bottom segment?

If so, can any single segment be part paternal, part maternal?

Please help me back on track...

enter image description here

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  • Would help to know what your relationship is to the two people and how the two people are related to each other if they are. Also how many cM are the two segments and the triangulated area.
    – lkessler
    Jan 18, 2022 at 18:13

2 Answers 2

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To answer the last question first: Absolutely, it is possible.

Consider that left edge of the circled area. The upper pink segment could be maternal on the left and paternal on the right (or v/v). Ditto for the lower segment (or v/v). That is, each "segment" is in fact two segments, the left maternal and the right paternal (or v/v).

"Oh, but it is so unlikely that the recombination would happen at exactly that point!" you say. And you would be right. But the likelihood is significantly reduced by the fact that the [unseen] "boundary" point need not be precise. Some slight fuzziness is allowed for in comparisons. But much more than that: though it is unlikely for the break to happen in that way, there are a lot of people. Billions. It could simply be that you are the one to whom this happened.

There are other reasons as well, but this could explain what is happening here.

Going back to the assumption that this is not the case (since the above is in fact quite unlikely), consider that comparison is not impeccable. Your DNA could almost match A and be considered one, ditto for the other - almost match B, but when comparing the two to each other, A and B are not close enough.

Third possible reason: there is actually no match, and the circled segment is short enough to be a false positive, though one maternal and one paternal. In other words, an unhappy accident that is leading you astray.

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I see two possible reasons:

  1. You could be related to the top person (red segment) two ways, one way on your father's side and one way on your mother's side. The red segment connected at the left of the triangulation may an overlapping match through the other parent.

  2. More likely: Remember that every true Identical by Descent (IBD) segment can have bits of random matches at either end due to the alleles of either of your parents matching the allele of either of your match's parents. Just like any non-IBD false match, these random bits can be up to 15 cM on either side of the IBD segment, although typically they will not be more than 7 cM, which is the minimum segment length normally considered to more likely be a true segment. So that bit of red connected at the left of the triangulation may be some random matching.

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