View Full Version : X-chromosome inheritance patterns

11-02-2016, 12:08 PM
An interesting information, since we can locate cousins at our X on 23andme, Family Treee DNA or Gedmatch.

The chart for a woman's X DNA inheritance shows that she inherits 25% (0%-50%) of her X DNA from her maternal grandmother. This means that on the average, a woman inherits 25% of her X DNA from her maternal grandmother, but could inherit as little as 0% or as much as 50%. The chart also shows that a woman inherits 50% (50%) of her X DNA from her paternal grandmother. This means that a woman inherits exactly 50% of her X DNA from her paternal grandmother. These charts ignore the possibility of Y crossover, which doesn't seem to contribute significantly to X chromosomes.

X chromosome inheritance for a woman:

5 generations chart:

6 generations chart:

X chromosome inheritance for a man:

5 generations chart:

6 generations chart:

11-03-2016, 03:21 PM
Yes, the X chromosome can often be quite useful when trying to narrow down how a person might be related.

I have two older brothers and three younger sisters. My eldest brother and eldest sister match each other entirely on the maternal copy of their X chromosome(s) -- obviously, the only copy my brother has. I also match them on this copy, except for the centromere and an area immediately surrounding it.

What this tells us is that the two of them each received a copy of their maternal X chromosome that either had identical recombination, or -- more likely -- no recombination at all. If no recombination, it means that it came from only one maternal grandparent.

So far, every person who shares a segment with these siblings has proven to be on our maternal grandfather's side. In addition, at the centromere I have a few relatives who match me on the X chromosome. They do not match these siblings, and they are on our maternal grandmother's side.

Anne Tydeselent
02-17-2017, 09:03 PM
I think the following X chart format is easier to understand than the circular type, however, I still find it confusing!


02-18-2017, 01:46 AM
One thing I think is helpful is to know that the number of possible X contributors in each generation increases according to the Fibonacci sequence: 1, 1, 2, 3, 5, 8, 13, 21, etc.

If you're male, you start with the 2nd number in the series -- that is, you only have one possible X contributor in your parents' generation, your mother. Females start with the 3rd number in the series, since both your father and mother contribute a copy of the X chromosome.

Each successive number is the sum of the two previous numbers, so it's pretty easy to figure out what Fabonacci number applies to any given generation -- at least within a genealogical time frame. You can also easily find a table.

What's truly amazing is to see just how small the pool of potential X contributors gets in just a few generations, compared to the total number of genealogical ancestors -- with or without allowing for pedigree collapse.

But the thing to keep in mind that once you get past your mother if you're male -- or your mother and your father's mother if you're female -- all of these potential X contributors are just that: potential. For example, two of my siblings appear to have inherited an X chromosome that's 100% from our maternal grandfather. That means all of our maternal grandmother's potential X contributors can be ignored.

03-09-2017, 08:25 AM
The inheritance of X-chromosome DNA follows an interesting pattern, and you can only inherit this from a defined set of your ancestors.

Wing Genealogist
03-09-2017, 08:43 AM
Due to this unique, sex-biased inheritance pattern, X-DNA matches can go a lot further back in time than other autosomal DNA matches. I have only begun looking at both my autosomal DNA matches as well as my X-DNA matches. I have uploaded my autosomal DNA to Gedmatch and I note where the vast majority of my X-DNA matches share no other autosomal DNA with me.

03-09-2017, 01:25 PM
Some musings about the X chromosome ...

As a practical matter, in each generation in which the X chromosome has any crossovers at all, it's likely to only be one or two. Occasionally, it might be three or four. The number of segments will equal the number of crossovers, plus one.

As an example, the X chromosome my mother passed on to my brother Bernie appears to have had no crossovers. Therefore it consists of just one segment, all from the X chromosome our mother inherited from her father -- which in turn is from his mother. How much is from my great grandmother's father, and how much from my great grandmother's mother is not yet known. Still, the segments from one or both are likely to be very large, since the chromosome probably didn't experience more than one or two crossovers.

In my case, there appear to have been crossovers somewhere around 41,061,863 and 71,279,596. This means my X chromosome consists of three segments from my grandparents' generation. The first and third are from my maternal grandfather; the second is from my grandmother. The two from my grandfather, of course, are from his mother; the one from my grandmother could be from either her mother or her father, but because it's only around 20 cM or so in length, it is quite likely to only have come from one of them.

My youngest sister CJ inherited more of our grandmother's X chromosome than I did. This also means she's more likely than I am to have inherited X chromosome segments from both of our grandmothers parents. Interestingly, she inherited a segment which 23andMe believes to be Native American. While the rest of us each have several such segments on the autosomes, she's the only one of us to also have a Native American segment on the X chromosome.

Both maternal grandparents had a Native American ancestor in their X paths, but this part of my sister's X chromosome came from our grandmother. Therefore, it's most likely that it traces back to the mother of Magdalaine Pany Baudreau. Magdalaine's father Jean Baptiste Baudreau was French; her mother (whose name may have been Suzanne), was Native American (possibly Choctaw).

Now, we're talking about someone who is a 7th great grandmother to my sister, or nine generations before her. I don't know exactly how long this segment is, but it might be about 20 cM or so. Of course, this is just a guess, and it doesn't involve a comparison to anyone who actually shares the segment.

But remember, if the X chromosome only experiences an average of 1-2 crossovers per generation (and it sometimes has none), then even in an all female line in nine generations that will be no more than perhaps 18 crossovers total.

If those crossovers were evenly spaced -- and they probably are not -- you'd have 19 segments that were each about 10.3 cM in length. But, as I said, they probably are not evenly placed, so you might well have a segment from a 7th great grandparent which was much longer than this.

The best use of X chromosome matches, then, is probably just that it greatly decreases the number of ancestors who might have contributed the segment. [Because they can be from much further back than similar-sized autosomal segments.] If it's your only match with someone, the connection might be very remote. Also, all you really know immediately -- if you're male -- is that you're related through your mother. If you're female, you only know it isn't from your paternal grandfather.

The further back you look, however, the more likely it is that multiple segment matches actually have multiple common ancestors. I have two branches of my tree -- one among my maternal grandfather's ancestors, and one among my maternal grandmother's ancestors -- in which lines crossed more than once. This definitely complicates things!

Or if you really want complication, I've learned of one ancestor who fathered children not only with his wife, but with his wife's sister. Reportedly, the man had 14 sons and 6 daughters. He would not, of course, have passed on his X chromosome to any of his sons, but he would have passed it on to all of his daughters.

EDIT: Here's a contrast. You can potentially have as many as 512 7th great grandparents. How many of these, if you're male, might have contributed to your X chromosome? Only 55. (For my brother, however, since all of his X chromosome came from our maternal grandfather, the number is actually only 21.)