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X-match FTDNA

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  • mabrams
    replied
    I have a match with 17 cM autosomally and 57 cM of X (49 plus 8).

    We are 5th cousins. The 17 is more relevant, if even a bit generous for a 5th. And the 57 was unduly optimistic if I went by the cM, but it did help me narrow down the common ancestor (a Dutch NY woman born in 1739).

    ~~~
    23andMe reported as one combined total of DNA and estimated me at 2nd to 3rd cousin, which was much too close.

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  • mnx
    replied
    Thank you for the excellent explanation KATM , very much appreciated. I will continue to explore the kit in question in more detail.

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  • KATM
    replied
    For both The Riddler and mnx:
    Leaving aside for now the issue of the under 7 cM segments on any of the autosomal chromosomes:
    • In general, for the minimum size to be considered Inherited by Descent (IBD) for a segment match on the X, it should be at least twice as large as the minimum for an autosomal segment (1-22). So, if we use a minimum of 7-15 cM for autosomal chromosome segments, then we need to use a minimum of 15-30 cM for the X chromosome. Obviously both of you show above this minimum for your X matches.
    • If the tester is male, and has a good match with another male on the X chromosome, each can be sure that the matching X segment(s) came from his mother's side.*
    • If the tester is female, and has a good match with a male on the X chromosome, she can be sure that he got his X from his mother. As a female, though, she has two X-chromosomes, so in her tree there are more possible ancestors who passed on an X segment.*
    *BOTH the latter two cases above should be able to narrow down the possible ancestors who passed the X segment down. You can use an X chromosome inheritance chart showing the male inheritance pattern (plus female inheritance for women), and fill in your known ancestors to see which ones could have possibly passed down an X chromosome segment. These charts are available at the following web pages:Back to the small segments on autosomes 1-22:
    At FTDNA, you will only receive a match on the X chromosome if the match also matches you on the autosomes for a minimum amount ("matching threshold" determined by FTDNA; I think this chart still shows pretty much the method they use). Also, FTDNA will label a match as an "X-match," even if it is a very small segment. See the first bullet at top; if an X-match is less than 15 cM, it is likely to be not relevant.

    I think that matching on the X chromosome, with only a small amount of matching segments on the autosomal chromosomes, should not affect your search for a common ancestor. I had a similar X match at another testing site a few years ago, but we could not determine the common ancestor (she had not tested for genealogy reasons, and did not have much knowledge of her ancestors). But we could each narrow it down to one branch of our X-inheritance ancestry (maternal or paternal), and then at least I could determine which line of my ancestors was the likely one to have the common ancestor with her.
    Last edited by KATM; 9 July 2020, 08:08 AM.

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  • mnx
    replied
    I am a male and have a similar case with an autosomal match with a female kit on X. I have some additional chromosomal matches at 26cM in 9 segments with largest at 12cM as well with this kit.

    X total = 156.71445 ( 37.6439, 27.38302, 26.02706, 14.90346, 8.25051, 8.20967, 7.74231, 7.71207, 7.28279, 6.77403, 4.78563)

    Is this a strong match or does it indicate something else?

    Xmatch.jpg

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  • The Riddler
    started a topic X-match FTDNA

    X-match FTDNA

    Hello,

    its my daughters match (male) on FTDNA:

    how should i interprete this?

    autosomal match
    6,71cM
    5,96cM
    5,72cM

    X-match
    2x 29cM
    1x22
    1x17
    1x16
    1x11
    1x8
    1x6
    1x5
    1x1

    thanks much



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