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Confusion on X inheritance

Reader Judy is having some trouble figuring out what an X-chromosome match means at the third-party DNA website“I’ve read most of the documentation listed under the ‘DNA for Dummies’ link provided on the website,” she writes. “However, I’m still confused about how to interpret the results. For example, one brother matches ‘John Smith’ x-DNA with total segments = 84.7 cM (largest segment = 27.0 cM). They also match at Chromosome 8 but with only 11.5 cM and 2,484 SNPs. However, my other brother doesn’t match ‘John Smith’ x-DNA nor any chromosome at all. I thought males inherited 100% of their x-DNA from their mothers. I understand why both brothers would not match ‘John Smith’ equally but shouldn’t they both match ‘John Smith’ –- especially when one has a number as high as 84.7 cM?”

And, she continued, “Is there any documentation that explains what you can do with the results you get? Or, what steps to take next to identify whether someone may be related (‘if xxx, then check xxx’)? How does someone determine whether a ‘possible’ match, such as ‘John Smith’ in the above example, is worth investigating?”

Welcome to the wonderful world of DNA, also known as the “we’re still figuring things out” tool of 21st century genealogy.

Because we still don’t completely understand all the ins and outs of DNA inheritance, particularly what the odds are of a result like the one Judy describes here.

Let’s back up a minute and define the problem. There are three major types of DNA tests used for genealogy:

• the YDNA test, which looks at the DNA in the male gender-determinative chromosome and helps establish the direct paternal line — your father’s father’s father and so on;1

• the mitochondrial DNA (mtDNA) test, which looks at the DNA in a part of the cell called the mitochondria that both men and women inherit from their mothers but only women can pass on, so it helps establish the direct maternal line — your mother’s mother’s mother and so on;2 and

• the autosomal DNA test, which looks at the DNA in what are called the autosomes, the 22 pairs of chromosomes we all have containing DNA randomly jumbled and passed down equally from our mothers and fathers that helps us locate cousins3 because it contains segments from many different ancestors whose DNA, by chance, managed to survive that jumbling process (called recombination).4

In the process of doing the autosomal DNA test, we also get information about our X-chromosome. That’s the other gender-determinative chromosome. Men have one X-chromosome, inherited from their mothers; women have two X-chromosomes, one from their mothers, one from their fathers.5 Judy’s question is about this particular type of DNA result, and her confusion comes in part from the fact that X-chromosome inheritance doesn’t behave exactly the way you might expect.

We tend to think that whenever we get a type of DNA from one parent (the way a man gets his YDNA from his father or the way we all get mtDNA from our mothers), the DNA won’t change very much from generation to generation. And YDNA and mtDNA do behave that way: there are relatively few changes from generation to generation. And we understand that when we get DNA from both parents (the way we inherit autosomal DNA), it changes a lot from generation to generation because of recombination.

But the X-chromosome inheritance is different. It doesn’t work exactly like any of the other DNA types at all.

A man gets only one X chromosome, from his mother, so his X-DNA is only from his mother’s side of the family. There’s nothing inherited in the X-DNA from his father’s side at all. But when a mother passes on that X, she gives her child a random mix of the X-DNA she got from her mother and her father. So two brothers will each receive only one X chromosome, each from their mother, but because of the random mixing their X-DNA will not be the same. Here for example is a comparison of the X-DNA of two of my mother’s brothers — the orange shows where they share X-DNA:


A woman gets two X chromosomes, one from her mother that’s the same type of random mix that her brother got (meaning she won’t match her brother exactly on that chromosome either) — and one from her father. And since her father doesn’t have another X chromosome for his X to recombine with when he passes it on, what he gives to his daughter is exactly the same X he got from his mother. And that’s where the random mixing took place: at his mother’s level.

So brothers and sisters will show very different X-DNA results, as this comparison of one uncle’s X-DNA to his brother (in orange) and his sisters (in blue and green):


And what that means is that the whole pattern is different from other DNA types: who we inherited the X-DNA from and how it’s randomly mixed. Which affects who we match.

And as more and more of us see more and more results on the X-chromosome side, we’re also seeing that sometimes we don’t get as much of that random jumbling — the recombination — in the X-chromosome as we’re used to seeing in the autosomes.6 And that may very well be the explanation for what Judy’s seeing here: one brother with a very big match to another person in the X-chromosome, and the other with no shared X-DNA at all.

Because what Judy’s seeing here is one very big segment from one maternal grandparent’s line that one brother inherited — and the other brother didn’t. Looking back at the first image above, you can see that it’s entirely possible for one brother to share a very big segment of X-DNA with a match — and for the other brother not to share any X-DNA with that match at all.

The takeaway here on this issue: just because both men inherit from one parent doesn’t mean you can expect they’ll both match a third person — or even each other very much — on the X-chromosome. It’s a different beastie.

But what about that second question — how do we know when a match is worth investigating?

The bottom line here is that we want to look at matches where the shared DNA comes through inheritance from a shared ancestor — a phenomenon called identical by descent or IBD.7 That’s by way of contrast from segments where we match someone else purely by random chance, called identical by state or IBS.8

And there we’re playing the odds. The frequently asked questions posted on the DNA-Newbie mail list offers the following as a guide to whether a single autosomal DNA segment measured in units called cM, or centiMorgans,9 is likely to turn out to be IBD (shared inheritance from an ancestor) or IBS (random chance):

11 cM or greater matching segment: more than 99% IBD, less than 1% IBS
10 cM matching segment: 99% IBD, 1% IBS
9 cM matching segment: 80% IBD, 20% IBS
8 cM matching segment: 50% IBD, 50% IBS
7 cM matching segment: 30% IBD, 70% IBS
6 cM matching segment: 20% IBD, 80% IBS
5 cM matching segment: 5% IBD, 95% IBS
4 cM matching segment: about 1% IBD, about 99% IBS10

The more segments you share, and the larger the segments, the more likely you and a match share a common ancestor.

The fact that only one brother has the match and not the other could push it back in time to a point where the common ancestor may be hard to identify. But still, combining an autosomal segment larger than 11 cM and a huge X-DNA segment from the one brother, that match is likely to be worth investigating.

And, really, what do we have to lose other than some time spent pursuing a genealogical question?


  1. See ISOGG Wiki (, “Y chromosome DNA tests,” rev. 7 Dec 2013.
  2. See ISOGG Wiki (, “Mitochondrial DNA tests,” rev. 7 Dec 2013.
  3. See ISOGG Wiki (, “Autosomal DNA,” rev. 1 Feb 2014.
  4. See ISOGG Wiki (, “Recombination,” rev. 1 Feb 2014.
  5. See ISOGG Wiki (, “X-chromosome,” rev. 20 July 2013.
  6. See e.g. Kitty Cooper, “What does shared X DNA really mean?,” Kitty Cooper’s Blog, posted 6 Jan 2014 ( : accessed 1 Mar 2014).
  7. See ISOGG Wiki (, “Identical By Descent segment,” rev. 1 Feb 2014.
  8. See ISOGG Wiki (, “Identical by state,” rev. 12 Nov 2013.
  9. See ISOGG Wiki (, “CentiMorgan,” rev. 1 Feb 2014.
  10. Kitty Munson Cooper, “Weekly FAQ,” DNA-Newbie list, posted 28 Feb 2014 ( : accessed 1 Mar 2014).
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