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Percentage DNA Match with Person whose Parents were First Cousins

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  • Percentage DNA Match with Person whose Parents were First Cousins

    Hello, I am hoping someone can take a look at the below diagram. I'm wondering what would be the expected percentage of DNA shared (approximately) between two people, one of whom's parents were first cousins. E.g. what would the percentage shared roughly be between Michelle and Patrick. Patrick is twice descended from Samuel and Helen, but Michelle is not. Regards Bob.
    hypothetical1 - Copy.bmp



  • #2
    At a guess, you'd potentially show about a generation closer than you should normally appear to be, so a 2nd Counsin Once Removed might look like a 1st Cousin Once removed instead. Although with inheritance being random they could still turn up in the 2nd Cousin Once removed Range while their siblings turn out a bit closer.

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    • #3
      Normally they'd be 2nd cousins once removed sharing around 0 – 316 cM with an average of 123 cM, but since Patrick has a high amount of endogamy in his ancestry, it's probably going to be more than that. How much more is difficult to say since there isn't really data available on specific types of endogamy like this. It looks like Richard and Wendy are almost double cousins (are the two Williams the same the person?) or at least 1st cousins, so Patrick would I guess be his own 2nd cousin. So I would estimate roughly double the amount - about 250 cM on average or a max of 630 cM, probably more if the two Williams are the same person.

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      • #4
        I am assuming Samuel/Helen is the same couple, which is why they're highlighted, and he said it was a 1C marriage.

        Generally speaking that should play out like double-cousins on steroids. And I know my own double-cousin matches tend to show up a LOT closer than the ones who aren't(but not always). Although I haven't attempted to quantify it due to limited sample sizes. Even with two such events on the same line on successive generations.

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        • #5
          So I decided to wander over to Ancestry and try to track down the other set of matches which would be more relevant to this question, rather than the double cousins.

          Other descendant branches of my family has a few cases of 1st or 2nd cousin marriages in their family tree.

          3C match, their grandparents were 2C to each other(note: we're "a generation closer" to one of these persons than the other; so they're also a 4th cousin):
          Myself: 102cm, 4 segments
          My brother: 40.6cm, 4 segments
          My mother(2C1R): 129.9cm 7 segments
          My Aunt(2C1R): 118cm, 6 segments

          But to further confuse things, on the line that we're closer to, they have a 2nd cousin that I am a 3rd cousin to, and they're not on an inter-married line:
          Myself: 105cm, 5 segments.
          My brother: 14.7cm, 1 segment.
          My Mother(2C1R): 118.9cm, 6 segments.
          My Aunt(2C1R): 62cm, 4 segments.

          (According to the shared CM project, a 2C1R averages 123cm, but can fall in a range from 0cm to 316cm; 3C averages 74cm, and ranges from 0cm to 217cm)

          But given that 4th cousin relationships can not match at all, and the shared CM project claims an average of 35cm at 4th cousin, the "bump" from the additional common ancestry path may not work out to much, or could work out to quite a lot. As you have an example in such variation with the 61.4cm difference between the "doubled" match vs myself when compared to my brother. Or the other cousin from the closer of the two lines where that difference between them and my brother jumped to 80.3cm out of 105cm possible(or 118.9cm possible if you reference from our mother). Likewise, my mother and her sister shows how things can vary wildly even in the same generation from a step closer. With the difference ranging from 11.9cm on the "doubled" match, vs a 56.9cm difference on the single line match.

          There IS a set of 1st cousins who married in that part of the tree who also share the same MRCA between themselves and me, but I guess they either haven't DNA tested yet, or I didn't dig far enough to find them before I decided to move on to other things. But it seems that once you get out past the 2nd cousin level, it starts to shift into "noise level" differences, although they do have a better chance of beating the average, for reasons that should be evident enough.

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          • #6
            Thank you for your responses. Michelle and Patrick would be 2nd Cousins Once Removed if Patrick's parents weren't 1st cousins. But they are 1st cousins. Which means Patrick and Michelle are doubly related. So they must be Double 2nd Cousins Once Removed. A 2C1R should expect on average to share approx 1.5% DNA. Therefore a Double2C1R should expect 2x1.5% or roughly 3%DNA shared.

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            • #7
              If only it worked that way. The issue you have comes back to the Shared CM Project's result set:

              "According to the shared CM project, a 2C1R averages 123cm, but can fall in a range from 0cm to 316cm"

              With the range being 0 to 316 cm, and the average being 123, a "doubled" average result would be about 250 cm, well inside the normal boundaries for a 2C1R match. In my own example, looking at two such matches, one of which had a (not quite) "doubled" path, and one with a single patch, the relationship with the extra path barely came above the average for the Shared CM Project for my Mother, and didn't clear the average for my aunt.

              However, the biggest takeaway is the range starts at 0cm, so a "double 2nd cousin, once removed" may turn out to be completely indistinguishable from a normal 2C1R. At least on a 1:1 test. If you tested the parent on that line and/or you siblings, any aunts/uncles on that related line, and/or other cousins on that line, chances are some of those test results could fall outside of normal bounds(above 316cm in this case). But that doesn't mean most of them will do so. Inheritance is random, and random is random. Some family bloodlines will have more in common on a certain (genetic) bloodline than other (genetic) bloodlines will.
              Last edited by bartarl260; 21st February 2019, 03:07 PM.

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              • #8
                That's true. Whenever the ranges overlap, we have to seek additional evidence, until we have enough to rule out the alternatives. The more relatives who can be tested, the better the evidence will probably be, but in some cases we may not be able to solve the problem conclusively. Uncertainty is part of science and also of genealogy! But we keep digging!

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                • #9
                  bartarl260: "Some family blood lines will have more in common on a certain bloodline than other bloodlines" If this means some bloodlines will/can share more, or less cMs, I agree, and have seen this with one of my lines. Therefore, I have learned to lower the threshold when looking for matches on this line. I have never heard this commented upon. Thanks!
                  Last edited by Biblioteque; 21st February 2019, 05:21 PM.

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                  • #10
                    Originally posted by Biblioteque View Post
                    bartarl260: "Some family blood lines will have more in common on a certain bloodline than other bloodlines" If this means some bloodlines will/can share more, or less cMs, I agree, and have seen this with one of my lines. Therefore, I have learned to lower the threshold when looking for matches on this line. I have never heard this commented upon. Thanks!
                    It's basically a bottle-neck, I can see hints of it play out as I look at aunts/uncles and my own nieces and nephews who have been tested, sometimes even with my own siblings. It becomes even more obvious when you're able to look at their own matching results directly. You can only inherit as much DNA relevant to a particular common bloodline as either one of your parents had to share, and that only defines the maximum you can get, the minimum can still be 0, particularly the further back you go. If their "share" of a certain genetic bloodline was below average then everybody downstream is going to have ripple effects from that.

                    There is of, course, the outlier possibility where "Dad" only picked up about 6.75%(~470cm) share of his great-Grandfather's DNA instead of the expected 12.5% average(881cm is the average as per sharedCM Project, bottom of the range is 464cm, top end is 1486cm), while one child picks up the full 6.75%(470cm) share and the other siblings pick up less than 2% from their 2x great grandfather, rather than the roughly 6.25% share the averages would suggest(or the 3.37% share suggested from the Father only having 6.75% to start with). Part of the challenge is figuring out where the "soft/weak spots" are on the genetic tree and how to work around/through it(mostly by finding someone who does have a larger share), but more specifically finding out who has "the more exceptional connections" to given family lines and how to capitalize on it.

                    It remains one thing which people have to be aware of that while "random inheritance is random" that randomness also often cascades down into the following generations. Such as the Shared CM Project showing that a particular family line can become virtually undetectable on some autosomal tests as early as 2C1R. While others have evidently managed to have enough DNA from one particular line stick around such that they could identify 8th cousins through atDNA.

                    Because of these "bottleneck" type events(if one line was "squeezed out" that means other lines are better represented than is typical), the importance of testing the larger family groups cannot be understated. Its why testing aunts/uncles and more distant cousins is important. Everybody holds different pieces of the puzzle in addition to the ones that are common to one another.

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