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My Bro & I have different cousins - help me understand please

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  • My Bro & I have different cousins - help me understand please

    I think I sort of know the answer to my question, but as I am not sure I am posting for confirmation or clarification and hopefully explanation. I have a pretty decent education but I feel very stupid dealing with dna. I feel like it is going right over my head.

    My brother and I both did FF and I manage both of our accounts. He and I are full siblings as we share 2443 cm. Not being well versed in this science I consulted this board to see what the experts had to say regarding my relationship with my brother because there was a question (actually a paranoid concern I had) that we may have been half siblings – but my doubts were allayed with the responses I received.

    OK, so here is what is going on. It seems that my brother has a dramatically different set of cousins than I do! We do have some matches in common, but those few that we do are … different.

    For instance, the third match on my list is a 2nd cousin with whom I share 103 cm. We have been in contact and have found our common ancestor which is on my father’s mother’s side. My brother also has this individual on his match list, however, she is on his page 2 and they share 67 cm.

    My brother has a 3rd cousin match, 8th individual on his list (80 shared cm) who is a known third cousin. We share third great grandparents in common BUT this person does not even show up on my match list AT ALL!

    I could give more examples but I won’t.

    I do understand that our matches are not going to be the same because of one of us getting more dna from one parent and the other more from another, but, we DID get some dna from BOTH of our parents so why does my brother have matches (close matches) that are not even showing up for me at all. I’m confused.

  • #2
    Full sibling only share 50% of their DNA.

    If you upload your and your sibling's raw DNA to GEDmatch you can see exactly how you full and half match DNA.

    My brother and I are full siblings. I have 987 matches to his 745 and we share only 275.

    I was expecting to share half of the 745 matches, but all our matches are worse than 3rd cousins so it must be below 50%.

    Look at the ISOGG cousin statistics https://isogg.org/wiki/Cousin_statistics.

    Only 90% odds that a 3rd cousin matches either of you and 50% for 4th cousins. So the odds of cousins matching both siblings drop fast.

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    • #3
      Thanks for the response. I have had our data on gedmatch for a while and looked at it again after reading your post. I see what you mean about full and half matches. I'm confused as to what they mean or perhaps how they will help me better understand.
      I appreciate your reply.

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