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Father & daughter share same number of cMs & SNPs

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  • Father & daughter share same number of cMs & SNPs

    I have a match with a father and daughter. All three of us have the same number of cMs and SNPS on chromosome 1. Isn't it odd that a father and daughter share the same # of cMs and SNPS? Also, Daughter and I share a small amount on X chromosome. Any idea what all this implies?

  • #2
    DNA is passed on in chunks, it justs means that daughter inherited that section of her fathers maternal or paternal chromosome 1. It just all depends on how the parents maternal and paternal chromosomes recombine to form a single to to child. Many times a single segment can be passed on generation to generation intact, more so if it is under 20cM.

    As for X, if you match the father on X, you will also match every daughter of his with the same amount due to the fact that his entire X is passed on to daughters.
    if referring to matching daughter on X but not the father, then X segment is likely false, especially if the longest segment on X is under 10cM

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    • #3
      If the kits were on GEDmatch, I would first check to see that the father's and daughter's kits really did match at the level expected of parent and child, and that it was not a case of the same raw DNA data file being uploaded twice! At least on GEDmatch, that happens all the time! It wouldn't be impossible for something like that to happen on FTDNA as well, if someone were very careless when preparing several kits at the same time. However, you didn't mention how strong the match is. If it is only a single segment, it is not particularly unlikely that it was passed intact from father to daughter. I would ignore the X match completely, at least to start with, unless the relationship with YOU is rather close: 2nd cousin or better. Without a close autosomal match, it is very unlikely that you will be able to make any sense at all of a weak X match.

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      • #4
        http://www.genie1.com.au/blog/63-x-dna

        And, in addition to prarielad and John's good responses, this link (and the links within) should help with your understanding of the X.
        Last edited by Biblioteque; 7th November 2018, 09:27 AM.

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        • #5
          That's not unusual at all. Recombination is random, so sometimes we inherit a certain segment from a parent in it's entirety, sometimes we inherit a portion of it (in varying amounts), and sometimes we don't inherit it at all. John makes a good point of making sure the same kit wasn't accidentally uploaded twice - but even at companies where uploads aren't an option, I see matches who share the same amount with me that they do with one of my parents. It's usually one segment that we share and I simply inherited that one segment in total.

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          • #6
            Originally posted by prairielad View Post
            As for X, if you match the father on X, you will also match every daughter of his with the same amount due to the fact that his entire X is passed on to daughters.
            if referring to matching daughter on X but not the father, then X segment is likely false, especially if the longest segment on X is under 10cM
            The X amount could vary a little bit between daughters, but they will be very close. When the 23rd Chromosome is XX recombination is possible, and the mother has two different ones to share with her daughters. As the matching algorithms are "greedy" in how they examine segments. It is possible that a different daughter might actually match even more closely (to you) than either her sister or her parents do. I have seen it happen in a handful of cases where I matched more closely on autosomal than either one of my parents did, even after ensuring we were comparing same test(/chip) to same test. The most extreme case I have experienced so far(autosomal in general) added nearly 10cms (!) to the match in total and .2cms to the largest segment.

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