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I'm confused. Maybe I misunderstand how xDNA works. I have a first cousin that doesn't share the same xDNA matches as me (we only have each other as common matches). Our atDNA appears to match on both our mother's parents sides of the family. (We are both males, we believe that our mothers are full siblings.)

Why aren't our xDNA matches the same?

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  1. I don't like calling it xDNA. I'd prefer to call it ChrX (chromosome X).

  2. So. The situation:

       [♂   G.Father  ]==[♀   G.Mother  ]
             XY                XX

    [♀ Your mother ] [♀ Your aunt ] XX XX

    [♂ YOU ] [♂ Your cousin ] XY XY

    • You share 100% of ChrX with your mother
    • Your cousin shares 100% of his ChrX with his mother (your aunt)
    • Your mother shares 100% of ChrX with her mother, and 100% of her other ChrX with her father
    • and your aunt shares 100% of ChrX with her mother, and 100% of her other ChrX with her father
    • in addition, your mother and your aunt almost always get a different combination of their mother's two X chromosomes from her.
    • but you may share with your granny (mother's mother) any part of ChrX in range from 0-100% because the rest of yours is from your mother's father.
    • the same applies to your cousin
    • Let's imagine that you got your X ONLY from grandfather and your cousin got his X from his grandmother ONLY. So your case can be real.
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    As George explains, you and your cousin both get some of the same segments of your X from the same maternal grandparent and some from the other maternal grandparent. As a result, your X matches are different than your cousin's. There is on average a 13% chance that 1st cousins will not match at all on their X chromosome. For more about how the X gets passed down, see: beholdgenealogy.com/blog/?p=1868 – lkessler Apr 6 '18 at 14:31
  • Let me clarify. In point 5 you write: "In addition, your mother and your aunt get a different combination...". So there is a recombination of the two ChrX that G. Mother has, when the new ChrX is created for my mother and aunt? (I didn't realize this happened. I thought they would receive only one of G. Mother's ChrX.) – TJinBC Apr 7 '18 at 0:59
  • Yes, you understand recombination correctly. But this process has some probability. Regarding percents you are not correct. There are chances that you got your X from your grandfather unchanged, or you got your X completely from one of both X from your grandmother, or any combination of X from one X of your grandfather and two X of your grandmother. It is not straightforward as inheritance of mtDNA or yDNA. And a little bit more complex than inheritance of atDNA – George Gaál Apr 7 '18 at 10:20
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    I read (multiple times) the link that lkessler gave in the first comment and that has given me a good idea of the probabilities involved with ChrX inheritance. Thanks to you both for your feedback. – TJinBC Apr 7 '18 at 23:32

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