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Need help understanding non-match and match with mother and daughter pair

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  • Need help understanding non-match and match with mother and daughter pair

    How is it possible for me (a female) to match a proven 3rd cousin (also female) on an X-Chromosome segment (3.61 cM) and not also match the 3rd cousin’s mother (my proven 2nd cousin once removed) on the X-Chromosome? I’m having trouble understanding how I could have that matching 3.61 cM segment with my 3rd cousin unless she inherited it from her mother, in which case I'd expect the mother to match me also. Our paper trail shows that I’m related to both of them (through the 3rd cousin's and my mutual 2nd-great-grandparents, and that I'm not related to the 3rd cousin’s father (the 2nd cousin’s husband). I wouldn’t have found it surprising if I had matched the 2nd cousin once removed (my closer relative) but not matched her daughter on that segment, but I’m not visualizing how it’s possible to match the daughter but not her mother.

    In case it helps someone answer, the mutual 2nd-great-grandparents are my mother's mother's father's parents; they are the 3rd cousin's mother's (the 2nd cousin once removed's) mother's mother's parents.

    TIA for any help in understanding this.

  • #2
    A half-identical match of only 3.6cM has a high probability of being a false positive, regardless of chromosome.

    In other words, both you and your match probably share ancestral populations and it just so happens that parts of your and her's X chromosomes are full of very small segments that are common in your ancestral populations, and it just so happened that some of these lined up to get you to 3.6cM "match".