Help me collect X chromosome data

A new approach to problem solving is to appeal to social media. So here is my request to “crowd” collect statistics about X chromosome matches. If you have multiple close family members DNA tested then please help me by filling out this form for each relationship at

XcollectingI have already put this request on DNA_NEWBIES and facebook so the form is now, hopefully, well tested and I will list some of my preliminary results at the end of this post. I have to give credit to Blaine Bettinger for this data collection idea; he is collecting autosomal statistics this way, see my post about that and the results so far.

Now why did I decide to collect these statistics aside from my own curiosity? To help an adoptee of course.

Recently I got an interesting question on one of my X chromosome posts. I was asked if the matching X could determine whether the uncle or the aunt of a female first cousin was the parent of an adoptee. The autosomal match with yet another first cousin identifies the shared grandparents and the female cousin is descended from them via her father. Neither potential parent, the aunt or uncle, has any known progeny nor are they available to be tested. Well?

My initial reaction was no way, X is too weird and undependable; but then I thought about it some more.

X inherited from grandma

X inherited from grandma

Since men only get one X chromosome, they pass the one they got from their mother intact to their daughters. Thus this female cousin has an unrecombined X from her Dad, half of her grandmother’s X. Therefore if this X chromosome match looks look like two brothers or a brother and a sister, it would be the uncle who is the father of our adoptee.

But how much X DNA do siblings, where at least one is a male, share? Is this predictable? Is it the same roughly 50% as they share in their autosomes?

In my experience, male or male/female siblings usually share about half to two thirds of the X so 90-120cM out of the total of 196cM (at Gedmatch and Family Tree DNA) or 182cM (at 23andme) with either one or two recombination points

Here is the match:

Chr Start Location End Location Centimorgans (cM) SNPs











The total shared cMs are about 70. This would suggest the aunt is the mother since that is not as much shared X DNA as would be expected if the uncle is the father. The expected amount of shared X between these first cousins, where one is getting her related X from her father and the other from her mother, is about 25% or about 65 cM as I show in my inheritance picture.

However I was not sure that the small sample of data that I have worked with was adequate for this conclusion. Also I wondered whether my observation that shared X usually exceeds the expected mathematical percentages was valid, so I decided to collect some more statistics via crowd sourcing.


Some preliminary statistcs on shared X

Here are a few preliminary statistics from about 50 and 100 data points. As you can see the expected amount for an uncle to his sister’s child fits our case.

The collected numbers for a brother and his sibling show that they usually share about half of the X which is statistically the expected result. And would also be the expected result if our adoptee’s father was the brother of this cousin’s father.

But these are just likely amounts and there are plenty of outliers. So I suggested that our adoptee test another sibling of the cousin to get us more input. Plus I am hoping for more data from my crowd sourcing effort.

The form is at at – thank you for helping out.

18 thoughts on “Help me collect X chromosome data

Click here to add your thoughts at the end of the comments
  1. I’m going to fill this out for my family members who have tested, but you may want to add another field to the form to ask about other information that may be helpful. With all branches being Ashkenazic Jewish, the amount of shared DNA on the X chromosome will likely be skewed higher than would be expected.

    • In my experience, the endogamous nature of a population has little effect on very close relationships. That effect is seen in further relationships.
      You still get 50% from a parent.
      And what I am studying is very close relationships
      But you talked me into adding a checkbox for endogamy

  2. I’m in agreement with Lara. I will be filling out some forms to help with this, but believe your selections might need explanation. I will enter my sister’s daughter, but it’s my half sister, so would think it would be helpful to know that when compiling your data. So a comments box seems as if it would help. I’ll wait to see if you can supply that feature.

  3. GEDmatch has an X matching tool that uses the sort of logic you are following. I have found it of great assistance for matches that are further out. It shows who can’t be involved and puts probabilities on who might.

    • Yes that is their “Relationship Tree projection” – I will be sure to give them a copy of the results of my study to help with that and of course I need to try it out and blog about it, which I have not done yet

  4. I submitted all the permutations of my large family (5 brothers, 2 sisters, 6 1st cousins and 4 grandchildren) that matched your criteria for inclusion. Even though the family is heavily skewed to males, there were enough of we females to make it interesting.

  5. Hi Kitty,

    I just submitted several for you. Hopefully I remembered to check the endogamous box each time.

    My brother to me and to my daughter. My dad to his brother and to my daughter. My uncle to me.

  6. How do I appeal to social media? I have left questions on message boards before but have usually found the links to be old, no one responds or the response has not been helpful? I am a totally right brained person with not a logical bone in her body that needs to understand how I can use my DNA results to find my gg grandfathers parents. After searching for 4 years on the Prince surname, I am told that William Prince was actually fathered by a Rose and born to a Prince female. I have daddy’s DNA with ancestry, and also a Y-DNA descendant with family tree and gedmatch. Even the websites help line don’t seem to have anyone that can interpret the findings so that you know what you are looking for. Any ideas? Lisa Montgomery

  7. Lisa –
    There are a number of great facebook groups for genetic genealogy try this one:
    There is a steep learning curve with this stuff. Test with 23andme also, get more cousins to test (more data can help with this), and perhaps take a class at
    Has the Y descendant done a Y test at ftDNA?
    You could of course pay some one to help you, perhaps an hour of someone’s time would do it … but learn more first perhaps

  8. I would love to help, but my closest tested is a known second cousin. I don’t even see many names that I recognize at FTDNA and even less at Gedmatch. Sounds like a good study though.

  9. I could provide x data for me and mom via ftdna, and my paternal half-sister via ancestry. would that help your project?

  10. Kitty,
    I see your X form, however, I do not see where you ask for the size of the X on that form. Did I miss it or do you only want those that have some amount of X matching, no matter how much?
    Thanks for all you do. This is great work!

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