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I have found a second cousin and have narrowed her down to being the granddaughter of my paternal Grandmother's only sibling (a sister). My new second cousin is adopted and excited to finally know about her birth family. I was wondering without opening a can of worms and disrupting folks is there a way we can figure out if we are related via her birth mom or father? My father (would be her parent's first cousin) took a DNA test and doesn't share an X with her (I don't either). Are we just stuck at this point until someone from that family group takes a DNA test? or is is a good guess that it is the mother?

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Why do you believe that you're related to her through your father's mother? Why not through your father's father?

Plotting the X inheritance path, your proposed relationship path means that you and your father should share some amount of X with this new second cousin. Because you don't, I do not believe that this is the appropriate connection.

My reasoning:

  • You inherited an X chromosome from your father. Your father inherited his X chromosome from his mother (a recombination of her two X chromosomes). His mother inherited one in-tact X chromosome from her father, and one X chromosome from her mother (a recombination of her two X chromosomes).
  • Your new second cousin inherited an X chromosome from her parent (an in-tact X paternally, a recombined X maternally). Her parent inherited an X chromosome from their mother (a recombination of their mother's two X chromosomes). Their mother inherited one in-tact X chromosome from her father, and one X chromosome from her mother (a recombination of her two X chromosomes).
  • If the two grandmothers are (full) sisters, one X chromosome is identical (the one from their father), and the X chromosome from their mother should have some shared regions.
  • Thus, regardless if your new cousin is related through her bio-mother or bio-father, she would have inherited X DNA from her grandmother, and would share some X DNA with you and your father.

Are you positive that it's a second cousin relationship? Within that same shared DNA range, you're also looking at a potential first cousin twice removed (1C2R) or a half first cousin once removed (half-1C1R) to you.

Check the current adoption laws in the state where your new cousin was born/adopted (see here: American Adoption Congress: State Overview). Some states are opening original birth records to adoptees. Contacting the agency she was adopted through may also get her the non-ID info of her biological parents; sometimes that's enough information to verify suspicions (and sometimes the information is entirely falsified). Some states/agencies will also attempt to contact the biological parents and see if they want contact with the adoptee.

Some other resources:


The above was written yesterday (17 May 2018). I wasn't sure where my 'Genetic Genealogy in Practice' (Blaine Bettinger and Debbie Parker Wayne; NGS 2016) book was yesterday, but I found it today and reviewed the chapter on X DNA.

It is potentially possible that you and your dad wouldn't share X DNA with this match. A mother's X chromosomes don't necessarily recombine before being passed on (pp 102-3). This basically affects every stage of my reasoning above. To reiterate, yesterday, I was assuming the X chromosomes always recombine, which is apparently not always the case.

This doesn't change the fact that it's still not possible to tell which biological parent your new cousin is related to you through. Your new cousin needs some additional close relatives to test, but at least she can separate most of her maternal and paternal matches, even if she doesn't know which is which.

I would still recommend exploring the other potential relationships (1C2R, half-1C1R). If your father has no half-siblings, you can rule out the half-1C1R.

Additionally, is your father aware of this cousin? Have you discussed with him the implications of talking to his three maternal first cousins about a child one of them gave up? Yes, it's possible that it could cause discord, but it could also be something his cousin has been hoping for. You and your father know his family best.

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