The Nexus of X’s

This post is about the X Chromosome. As you likely know, we all have 2 sets of Chromosomes – one from each of our parents. These Chromosomes are numbered 1 though 23. 1 though 22 are the Autosomal Chromosomes. The 23rd is sometimes called the sex chromosome. Women get 2 X chromosomes: one from their mother and one from their father. Men get an X chromosome from their mother and an Y from their father. And it is important to note that the man does not pass down any X chromosome to his son.

There is some good information out there on the X Chromosome. One blog is called That Unruly X by Roberta Estes at DNAExplained.

I have had questions on what the X chromosome matches mean. In one way, the matches are like any other autosomal match. In another way, they are different. The way they are the same is that they should indicate a common ancestor at some point.

A Real Life Situation with My Sister Sharon

Let’s look at a real life situation. My sister Sharon tested recently. If I go to Gedmatch.com, I can run her DNA in a utility called ‘One to many’ matches.

One to Many

I entered Sharon’s kit # and checked the X radio button so I would get her largest X match. This gave me Sharon’s matches at Gedmatch. Unfortunately, these are sorted by ‘Gen’

X DNA Sort

Gen is the number of Generations that Gedmatch thinks your match is from you. I can tell this by the red arrow (triangle) pointing up under Autosomal. This means the smallest number of Generations are sorted first, which are the closest or most important matches. I want to sort by by X-DNA ‘largest cM’. I did that and got this for my sister’s top 4 matches:

Top X Matches for Sharon

Karen and Sharon’s Big X Match

3 of the 4 people I know. Heidi is my other sister and Gladys is my mom. They share all their X Chromosome with Sharon. Joel is me. But who is karen? And why does she share more X Chromosome with my sister than I do? Let’s figure it out together. On discussion groups I have been challenged by the fact that it is impossible to tell where a match comes from by an X Chromosome match and no other autosomal match. Let’s assume that this is true. However, in this case, karen matches Sharon on an autosomal Chromosome also. These 2 matches may represent different ancestors shared by karen and Sharon. However, these 2 matches should both be on Sharon’s maternal side or paternal side.

Which Side are You On?

As with all matches it is important to know if these matches are Maternal or Paternal. All of your autosomal matches will be one of the 2. For example, my father’s mother is a Frazer. That means I should be looking for Frazers on my paternal matches. If I find someone who matches my mother and has a Frazer in their ancestry, that match is likely coincidental and not where the DNA match is. So is karen a paternal match or maternal? If these were my matches, my guess right away would be that karen would be a maternal match. How would I know? I only have the X Chromosome from my mother. Sharon on the other hand has 2 X Chromosomes: one from our mom and one from our dad.

Here is how karen and I look on Sharon’s FTDNA Chromosome browser.

Sharon Karen Joel X

The orange is my match with Sharon. The blue is karen’s 56 cM match with Sharon. There is a chunk of orange overlap above the blue. Does this mean that Sharon, karen and I match on the X Chromosome? No. Remember that I only got my X Chromosome from my mother. Karen is related on my father’s side where I didn’t get any X Chromosome. So while it may look like a match on the browser, it is not, as the orange represents my Maternal match with my sister and the blue represents my sister’s Paternal match with karen.

Where Does Karen Fit In?

In some of my other blogs, I have been able to fit in people into the Frazer Project I have been working on. Let see if karen also fits in. First, I’ll look at her autosomal match with Sharon. To do this, I click on the ‘A’ to get the details of the autosomal match between karen and Sharon. When I do that, I get this:

Karen and Sharon Chr9

Fortunately I have had my mom tested for DNA. With these results, I was able to phase her DNA using a gedmatch utility. This results in Sharon having 2 additional kits: a paternal and a maternal kit. I ran Sharon’s Paternal Phased Kit against karen and got a match slightly smaller than the one above. This proves that the match is on the paternal side.

Then I checked karen against other testers in the Frazer project. She matches Jane and Prudence who are both known Frazer descendants. Interestingly, Jane had been in touch with this person prior to DNA testing concerning karen’s genealogy through more traditional methods. Jane and Prudence represent 2 different old related Frazer lines going back to the early 1700’s. In addition, I noted that karen had an ancestral tree at FTDNA with an Ann ‘Fraizer’ in it. Now there is no triangulation between karen, Jane and Prudence meaning no proof by the DNA that there is a common Frazer. However, the evidence of karen matching 3 Frazer descendants from by DNA and having a Fraizer in the same general area of Ireland is good circumstantial evidence.

Back to the X

All of this is interesting, but what does it tell us about the X match? To me, it says that the X match is in the same general area as the autosomal match. That means in the area of Frazers – so this could be from a Frazer or Frazer spouse. We know that the X Match is from the Sharon’s Paternal Side. I also double checked that by running an X ‘one to one’ match with Sharon’s paternally phased kit and got the same X match with karen. Sharon’s maternally phased kit did not match karen.

Here is what my dad’s X inheritance looks like:

Dad's X Inheritance

Here the pink and blue areas are the only areas Sharon could have an X match through my dad who is James Frazer Hartley. Unfortunately, these pink and blue areas go to places where it is difficult to find ancestors. In general, that would be on the maternal sides of lines where historically not as much information is not always available. Some notes from the above X Inheritance chart:

  • Sharon’s X match with karen could not be through our 2nd Great Grandfather George William Frazer.
  • The match could be through George Frazer’s wife who was Margaret McMaster
  • Margaret McMaster had a Frazer grandmother. It is possible that an X match could be through her.
  • The X chromosome does not recombine as much as autosomal DNA. This means a larger intact segment can travel down through the ages. This helps explain the large X match between karen and Sharon.
  • The percentages shown above are theoretical averages. The real amounts could be much larger or much smaller. In our example, Sharon got 25% of her X Chromosome from a part of the chart that shows that the theoretical amount she would’ve gotten would be perhaps 6-12% or less.
  • Also, the X Chromosome that my sisters got from my dad was the same that he got from his mom. It doesn’t even have a chance to recombine.
  • The match could be through the Clarke line as they were in the same general part of Ireland, but I would tend to think that it is more likely that the match would be on the McMaster/Frazer part of the chart.
  • My guess is that the autosomal portion of the match would be in the same general area as the X Chromosome match. Even though the shared ancestors represented by the autosomal match are probably not the same as the shared ancestors represented by the X Chromosome match, it would make sense to me that they would be nearby each other on the chart.

 

 

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