May 14, 2017

Understanding Your X Chromosome DNA

When it comes to DNA, I am either asked privately or see posts asking why Ancestry shows an individual further related than what the person appears on GEDMatch. This has to do with how Ancestry's Timber software analyzes the DNA file.  Timber has been known to remove centimorgans (cM) where chromosomes are bunched together (clustered) which can lead to removing relatives from your list.

I have seen this occur with people who are distant related but I have documentation that they are indeed related via records. This phenomenon isn't just my tree but many other people complaining about the same issue.

The best option is to download your raw file from Ancestry and upload it to GEDMatch to further analyze your DNA.  While learning to use GEDMatch may seem hard at first, it actually isn't. If you need help, there are many groups on Facebook, many blogs that will help you along the way, and plenty of YouTube videos.

There are also wiki pages associated to each of the options selected on their website.  Just remember that this website is free and ran by many volunteers.  There are additional tools available that you can use at the cost of  $10 per monthly.  However, learn more about the website before taking on too much.
Getting to what I want to discuss, X Chromosome, note that you should not be relying solely on X to identify relationship once you are in GEDMatch.  The individual should be matching you on other chromosomes. 

Lately I am seeing posts of "I have an X match", does that mean we match via a particular parent? The answer is yes and no.  It all depends if you are male or female. Below is an illustration of how we inherit X as a male and hope with the visual it will clarify the answer to the question.

Note, we are not discussing mitochondrial DNA (mtDNA) in this post; that is another discussion. This is to keep the discussion as simple as possible and the illustrations have been over simplified to help you along.



Males have a Y chromosome and a X chromosome.  Males can only inherit Y chromosome from their father and a combination X chromosome from their mother.

Females have two X chromosomes. Females inherit an exact copy of their father's mother's (paternal grandmother) combination X chromosome and a combination X chromosome from their mother. 


Combination X from your mother mean that you do not get an exact copy of one of her X chromosomes, it is just as random of inheritance as it is for your other 22 chromosomes (called autosomes).

So for example, I have two half sisters, we share the same mother.  Our shared amount of X chromosome DNA is different from one another. With one sister I share 94 cM and the other I share 64 cM. However, they are two full sisters, so they share an exact copy of their father's DNA and a portion of our mothers, between them they share 196 cM.

Below is an illustration of what I stated before, note that you can inherit many combinations from a mother but I left this very simple; you can click on the image to blow it up from a computer.

Males can inherit many different combinations from their mother and pass an exact copy down to only their daughters.  So every female is walking around with an exact copy of their father's mother's combined X DNA.




Most importantly, you should not assume if a person only matches you on X and not on any other chromosome, that you are related.  It is not a reliable way of determining relationship.  I hope this clarifies many questions on X. 

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