Saturday, January 21, 2017

Segmentology and the X chromosome- a theory

It isn't hard to find references to "sticky" DNA or inheritance of segments in blocks on blogs today. And almost all of the experts will tell you not to use the X chromosome for analysis and confirmation of relationships. I am not a hundred percent convinced that we should avoid it entirely though.

I don't know the reason for the affinity of some DNA to stick around in larger segments over several generations. Is it because within those SNPs is a sequencing that is more preferential, or is it a factor of genetic dominance? I really don't know. And maybe someday I will become more educated and understand the why better. For now though, I don't need to know the why.

There is definitely prevalence for large inheritance patterns in the first 22 chromosomes on both of my parents, mostly my mom, but what about the larger DNA segments on the X chromosome? Is there a way of looking at the amount of DNA and guessing the location of the generation it comes from? I am not sure, but I would like to try and find out.

The X chromosome is funny. For one thing, men only have one, so they pass what they receive entirely to their daughters. But the mom's well they can pass it along in all sort of ways. I have seen one analysis in one blog where one of the X was passed as whole for several generations. Which is why I think they say don't look at it. My daughter got 2/3 of her X chromosome from me from my father, and only 1/3 from my mother. Something I think that explains a great deal in why she has so much less Native DNA in admixtures. A lot of my mom's Native DNA is on her X chromosome.

Looking at my father's X, he (well we) share a rather large segment in common with several descendants of  our McCurdy family and a few others. I have been trying to pinpoint a common X donator between two of those others to give me a clue on the mother of my ancestor Barsheba Collins. How did I pick her? Well, if you don't know how to tell your X donators, there are some excellent blogs, but basically it goes like this.

If your female, your X donators are your mother and your paternal grandmother. If your male your donator is your mother. It goes back from there in the same pattern.

My father's mother was Evelyne Virginia Hardy. Her mother was Lula Bell Pyburn, and her paternal grandmother was Talula Johnson. Lula Bell's mother was Plina McCurdy. Her paternal grandmother was Mary Jane Chitty. I don't know Talula's mother, but her paternal grandmother was Elizabeth Parker. Plina McCurdy's mother was Amanda Beck, and her paternal grandmother was Barbara Sunday.

Each of the other McCurdy's share the same pattern back to Barbara Sunday, and only one of them has Amanda Beck. Which means for the side I know I have to look at Barbara Sunday. We don't know her paternal grandmother's full name, but she was born in Europe, which doesn't look like the likely candidate since the others all have trees firmly in America, most specifically Georgia. So Barbara Sunday's mother was Barsheba Collins. That's how I got to her.

Looking at the amount of DNA depending on the company, your X is around 196 cMs. The segment I am looking at is 40 cMs, so not a small segment. Which in theory should mean, it should be closer, well, in my theory anyway. So can I take that 40 cM and look at the amount and possibly look at a number of generations?

My mom and her father's first cousin share 89 cM on the X chromosome. In this case, the cousin is female, so her entire X chromosome from her father came from her mother, Margaret Trahern. Mom's entire X from her father came from Bonnie Adams, a daughter of Margaret Trahern.

Though the inheritance patterns stick around longer on the X, I would think that it may be reasonable to assume that siblings on average share 25-75 percent of the X, but that range in theory could be 0. In terms of cMs then 46-140 cM's, with the possiblity (rare?) of 196 cM's.

I am going to probably not make this very clear, but I propose that all of your first x donators be generation 1, then all of their x donators be generation 2, and so on. Instead of a deduction by 50 percent, since the rate of change is slower and only one parent recombines, I did 25 percent. I also added a 10 percent anomaly range because well, X inheritance is funny. I would be interested to know if this theory of ranges pans out in triangulated matches as an approximate average.

Generation Number for your X donator
Maternal (X donator)
Paternal (X donator)
Proposed range cM
Additional 10 percent for anomoly
1 See note
Father’s mother (female)
Grandmother and mothers paternal grandmother
Paternal grandmother’s mother and paternal grandmother’s paternal grandmother
Grandmothers mother and paternal grandmothers mother and her paternal grandmother
Paternal grandmothers mothers mother, and so on …
28-78.5 cM



  Note: There are rules for this range.
1. Sisters will share entirely one X chromosome that they inherit from their father. They can share a possibility of  none (in case of non recombination) to 100 percent on the Chromosome they get from their mother.
2. A brother and a sister would share none to 100 percent of their X from the mother.
3. Two Brothers also would share none to 100 percent of their X from the mother.
4.  Paternal females first cousins will always match their grandmother's X at 196 cM's but may not match each other at that high of a rate, (it would be the same as brother to brother).

I have edited this to include the clarification above and this explanation.
I was corrected on my chart by another researcher who cites non recombination as up to 3 percent in occurrence on the X chromosome. I have read several scientific articles however that show the recombination rate on the X is 0.8 percent with three hot spots where recombination is likely to occur. That is the reason I gave a proposed range of 25-75 as an Average for siblings. Someone way smarter than me gave a statistical math formula for the inheritance of the X. That blog is here.

 In the case of both my parents, the table holds true for mom's and for Dad's, it's accurate for their common ancestor, and most likely DNA contributor as the source. I looked into two more, one for each parent, and it would seem that the range is a good place to start looking. For example, my mom matches her mother's sister at 61 cM's which is within the range for 2 on the chart. Note due to recombination, I do not match my great Aunt.


  1. Just testing to see if my comment would post. Had troubles the last time I tried.

    1. Can't use my wordpress account

  2. Will this help you when my results come in?
    And what do you expect the results to be?