Announcement

Collapse
No announcement yet.

Mother/Son cM values on shared matches

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Mother/Son cM values on shared matches

    My mother and I share two matches with different cM values for longest block and total shared.

    Mother's Match A: LB = 23.76 cM, TS = 38.55 cM

    (mother's match A is listed as 3rd to 5th)

    Son's Match A: LB = 23.76 cM, TS = 45.17 ?

    (son's match A is listed as 2nd to 4th cousin?)

    Mother's Match B: LB = 19.33 cM, TS = 53.01 cM

    (mother's match B is listed as 2nd to 4th cousin)

    Son's Match B: LB = 20.54 cM, TS = 48.03 cM

    (son's match B is listed as 2nd to 4th cousin)

    Where I have trouble understanding is why with match A my total shared is MORE than my mother's, which I wouldn't have thought possible.
    Plus, I am listed as 2nd to 4th where my mother is 3rd to 5th cousin, which I also wouldn't have thought possible.

    In match B I have a longer segment than my mother, which I do think is possible and can understand why that might happen.

    But with match A, my mother can't possibly have less total shared. Doesn't make sense. I should inherit either the same amount or less, or at least that is what I would have thought intuitively. How can I inherit more than what my mother gave me?

    Thanks for your patience.
    Last edited by SMD; 25 April 2014, 06:58 PM. Reason: mistake

  • #2
    I have a clear example of this with my son, my wife and myself. I match my father's cousin, as you would expect from a 1st cousin once removed. My wife also matches my father's cousin, as does her first cousin. OK there is a 6th or 7th cousin relationship back in New England and it has persisted.

    So my son matches my father's cousin with DNA from both of his parents. And he has a longer matching segment that I do, thanks to a little extension of SNPs from his mother.

    Comment


    • #3
      Does my having a larger total shared for match A, than my mother, suggest that I am picking up extra portions of the same segment from both my maternal grandparent's side's, through this match's common ancestor to us all?

      Is that what you are suggesting might be a possibility here?

      Then, if that is possible, would that be possible for match B's longest segment being longer for me compared with my mother? Could the same suggestion apply to both match's in both longest and total shared, in my case?

      Comment


      • #4
        Yes. The test just recognizes that one of your two SNP values matches one of the two values of the other person. There is no way it can tell which parent a value came from. But since your parent tested you do know more.

        Comment


        • #5
          Well... that is unexpected.

          I'm still not clear how this could be possible. During recombination, the autosomal DNA of my mother's father and mother's mother would produce a specified total shared portion from this unknown ancestor, even if common to both of them. I could only inherit what my mother provided me with, or less. No?

          Are you then saying that I might have inherited my mother's father's portion, and my mother might have been left with my mother's mother's portion? Does that explain why mine is longer or larger than hers?

          But then I am having trouble imagining how that must work during recombination. It would suggest that my 45.17 cM portion from this unknown ancestor via my maternal grandfather was an altogether different portion than the 38.55 cM portion my maternal grandmother passed down to my mother. Am I on the right track? That is the only way I can see this making any sense.

          Also, since my mother is estimated as a 3rd to 5th, and I am 2nd to 4th to this unknown common ancestor, I assume then that I can safely say I am likely 2nd, 3rd, or 4th cousin, but not likely 5th cousin. It seems to me 5th cousin would be too far outside both relationship estimates taken together. My estimate seems to suggest a closer cousin relation than what my mother's estimate suggests? Is that a safe bet you think?

          Comment


          • #6
            Originally posted by SMD View Post
            Well... that is unexpected.

            I'm still not clear how this could be possible. During recombination, the autosomal DNA of my mother's father and mother's mother would produce a specified total shared portion from this unknown ancestor, even if common to both of them. I could only inherit what my mother provided me with, or less. No?
            Are you then saying that I might have inherited my mother's father's portion, and my mother might have been left with my mother's mother's portion? Does that explain why mine is longer or larger than hers?

            But then I am having trouble imagining how that must work during recombination. It would suggest that my 45.17 cM portion from this unknown ancestor via my maternal grandfather was an altogether different portion than the 38.55 cM portion my maternal grandmother passed down to my mother. Am I on the right track? That is the only way I can see this making any sense.

            Also, since my mother is estimated as a 3rd to 5th, and I am 2nd to 4th to this unknown common ancestor, I assume then that I can safely say I am likely 2nd, 3rd, or 4th cousin, but not likely 5th cousin. It seems to me 5th cousin would be too far outside both relationship estimates taken together. My estimate seems to suggest a closer cousin relation than what my mother's estimate suggests? Is that a safe bet you think?
            You have to remember we all have 2 of each chromosome, one from our mothers and one from our fathers.

            each position tested has two values (ie AG), one value from our maternal chromosome (ie G) and one value from our paternal chromosome (ie A). Matching algorithm can not differentiate which value goes to which chromosome(maternal or paternal), it just looks for blocks of single values in a row. Meaning it is creating a match by picking one of the two values you have and also picking one of the two values the match has. As long as it can create a continuous matching sequence, it will list that length.

            Tail ends of childs segments that are larger then parents are most likely due to this and are IBS segments(sections not shared by parent)

            Comment


            • #7
              I guess I'm not grasping how recombination and matching/sequencing actually works then. If you know of a webpage that explains these processes better, please let me know.

              Comment


              • #8
                Originally posted by SMD View Post
                My mother and I share two matches with different cM values for longest block and total shared.

                Mother's Match A: LB = 23.76 cM, TS = 38.55 cM
                Son's Match A: LB = 23.76 cM, TS = 45.17 cM

                Mother's Match B: LB = 19.33 cM, TS = 53.01 cM
                Son's Match B: LB = 20.54 cM, TS = 48.03 cM

                Where I have trouble understanding is why with match A my total shared is MORE than my mother's, which I wouldn't have thought possible. Plus, I am listed as 2nd to 4th where my mother is 3rd to 5th cousin, which I also wouldn't have thought possible.

                In match B I have a longer segment than my mother, which I do think is possible and can understand why that might happen.

                But with match A, my mother can't possibly have less total shared. Doesn't make sense. I should inherit either the same amount or less, or at least that is what I would have thought intuitively. How can I inherit more than what my mother gave me?

                Thanks for your patience.
                I was going to answer the last post in this thread but I thought I'd go back and start at the beginning.

                First.. (and foremost), you are looking at two issues here. One question is why do you match more than your mother and the second has to do with the prediction. So the first answer to your questions is simply that the prediction is based on the numbers. The computers do not know when segments are compounded, etc., and simply predict based on the numbers.

                If we take the predictions off the table and try to understand where the numbers came from that may help. First I see that in reality, Match A to both of you is by the same size longest block so that is telling me that segment is shared from one common ancestor between your mother and match A. The smaller segments could be from anywhere and in fact I would not pursue them until you find the largest matching segment connection first. The smaller segments could be due to some residual matching SNPs on your father's side, or test issues, or Match A's other parent's side to your father. They are really not large enough to consider in this case.

                Just to summarize, match A to your mother (and then to you) is from one common ancestor and could be anywhere from 5th - 9th cousin and those extra segments may just be "fooling" the predictor. I happens all the time. The extra 7 or 8 cM between the summed segments is not worth looking at and that is why the prediction is different between match A to you and to your mother.

                Next, match B. 19.33 and 20.54 is really not that much different for the longest block and is probably due to either test anomalies or a few paternal matching segments being added into your longest block match with B. Again there is an extra cM summed segment difference between the two only this time the larger sum is on your mother's match and that is normally what happens. So in this case, just a few cM different in the longest block, yet the expected extra sum on the parent's side. Again, these small differences will change how FTDNA predicts them.

                The fact is, these are normal differences to be expected when comparing a child and a parent to the same distant relative (over 5 generations back).

                So taking away the small differences, these matches both mean that you match A and B on your maternal side. The differences are either due to small matches from your father's side, or test anomalies and are not anything to worry about.

                All of these factors have to be taken into account for the FTDNA FF tests because FTDNA reports all small matching segments, not just the ones over 5 or 7cM.

                Matt.

                Comment


                • #9
                  Originally posted by SMD View Post
                  Does my having a larger total shared for match A, than my mother, suggest that I am picking up extra portions of the same segment from both my maternal grandparent's side's, through this match's common ancestor to us all?
                  I believe it suggests that your parents, rather than your mother's parents, have a common ancestor. And that you inherited DNA from this common ancestor from both your mother and your father. As a result, you share more DNA with this person than your mother.

                  Comment


                  • #10
                    Originally posted by mkdexter
                    Just to summarize, match A to your mother (and then to you) is from one common ancestor and could be anywhere from 5th - 9th cousin and those extra segments may just be "fooling" the predictor.
                    So there is no chance this match is any closer, between 2nd and 4th maybe? Or are you just saying, it could be farther out beyond 5th?

                    Comment


                    • #11
                      Originally posted by jonr View Post
                      I believe it suggests that your parents, rather than your mother's parents, have a common ancestor. And that you inherited DNA from this common ancestor from both your mother and your father. As a result, you share more DNA with this person than your mother.
                      That would be very odd, since my father's ancestry is French and my mother's is British. Any residual segments lasting that long would have to have come from way back in history.

                      There is a possibility that they share some Scandinavian connection, as my father has some recent Scandinavian ancestry, and I've just discovered recently that my mother has also... which came as a shock because we all believed she was 100% British. Yet according to my match results, both sides seem to have at least some Scandinavian matching going on. If you are correct, then that is likely where this commonality is coming from.

                      Comment

                      Working...
                      X