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Three Copies of a Chromosome?

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  • Three Copies of a Chromosome?

    A third cousin on my mother's mother's mother's side has DNA results at MyHeritage.
    I have been playing with DNAPainter, so checked his results there.

    On Chromosome 16, he matches me for a 10.6 cM segment in an area where I am matched by relatives on my mother's side and my father's side.
    On my mother's side by my mother's sister and also on my mother's side by a second cousin once removed and three second cousins -- all on my mother's father's side.
    On my father's side, I am matched there by a first cousin and for most of the distance by a third cousin.
    These relatives at FTDNA on my mother's side match me and each other there - so a match on my mother's father's side
    And the two relatives at FTDNA on my father's side match me and each other there.
    The two groups do not match each other.

    Having tried repeatedly to get him to transfer to FTDNA, I finally got permission and transferred the results from my mother's sister and my first cousin on my father's side to MyHeritage. It shows each of them matching me over that section of Chromosome 16 --
    But -- there is no triangulation there of me and that third cousin on my mother's mother's side with either
    my mother's sister or my first cousin on my father's side. (I did each set of three separately. He and I and my mother's sister have other areas we do triangulate)

    So -- does that mean I have three versions of Chromosome 16 or ???
    Last edited by loobster; 17 November 2020, 10:21 PM.

  • #2
    We can be fairly sure you don't have full "Trisomy 16" (Google search will be informative). However, it is possible to end up with SOME extra copies of chromosome 16 (but remember, they had to come from somewhere), or a copy of chromosome 16 that contains a duplicated section. It is also not out of the question that at least one person involved in your results has had a transfusion, organ transplant, etc., or even that the result is entirely bogus as a result of whatever molecular genetic circumstances cause spurious matches in "pile-up regions".

    One way to proceed would be to gather more evidence about the behavior of this particular segment, by transferring the kits to GEDmatch and using the tools on that site to compare all of the kits to every other one, individually, and also to search for additional shared matches involving the same segment.

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    • #3
      According to Wikipedia, "It is not possible for a child to be born alive with an extra copy of this chromosome present in all cells (full trisomy 16).[4] It is possible, however, for a child to be born alive with the mosaic form" and "Mosaic trisomy 16, a rare chromosomal disorder, is compatible with life, therefore a baby can be born alive"
      and following up with with footnote 9 gets "Conclusion: Diagnosis of trisomy 16 does not necessarily mean that the newborn has anatomical abnormalities."

      Seems clear my aunt's copy that I am matching her on is from her father (which would also be my mother's father) given the other relatives matching both of us there, and each other.

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