The X chromosome has always fascinated me. Whether it is trying to track down the family mild blue/green color blindness or just observing X inheritance patterns in my family and others.
The old farm house at Fatland farm, Halsnøy Island, June 2015
This morning I woke up all excited because I realized that a large piece of my X chromosome comes from my Norwegian 4th grandmother Metta Olsdatter Ve (Fatland) born 1729 on farm Fatland, likely in the house pictured here. She died in 1805 on farm Ve (or Wee) so made it into the 1801 census.
So how did I figure this X inheritance out? My Dad and I share 33.5 cMs of the X chromosome with my fourth cousin once removed Maria and her mother (so it is phased). They descend from a different wife of our common ancestor Aamund Bjørnsen Tvetden. So these 3326 SNPs of X located from 68M to 113M must come from him and therefore from his mother Metta.
No I did not share this insight with my husband when he brought me my morning coffee. I know better than to make his eyes glaze over … only other genetic genealogy aficionados would appreciate this, so I am sharing it here instead.
By the way I am still collecting X statistics, so click here for more on that and please contribute yours
Several of the Ve farms, Etne, Hordaland , June 2015
A new approach to problem solving is to appeal to social media. So here is my request to “crowd” collect statistics about X chromosome matches. If you have multiple close family members DNA tested then please help me by filling out this form for each relationship at http://goo.gl/forms/4294xjjhqq
I have already put this request on DNA_NEWBIES and facebook so the form is now, hopefully, well tested and I will list some of my preliminary results at the end of this post. I have to give credit to Blaine Bettinger for this data collection idea; he is collecting autosomal statistics this way, see my post about that and the results so far.
Now why did I decide to collect these statistics aside from my own curiosity? To help an adoptee of course.
“My half-sister and I have been tested to confirm half-siblings. Is there any way to know which parent we share” – this is a rephrasing of a question posed by a woman on the DNA-ADOPTION mailing list at Yahoo. A discussion of the problems of mtDNA ensued. Because the mtDNA haplogroups go back thousands of years, a match is no guarantee of a recent ancestor like a mother. So that information can only rule out the maternal side if there is a mismatch. Deeper mtDNA can get closer but still this is not certain territory yet.
However since they are both female, the X chromosome can give a definitive answer because their father(s) would pass his only X chromosome unchanged to each of them. A man has one X and one Y chromosome and a woman has two X chromosomes. Whether the father passes an X or a Y to his child determines its sex, but since neither is recombined the child gets exactly the same X or Y chromosome as the dad has.
Since a girl has two X chromosomes, one is from her dad. Two sisters with the same dad will therefore ALWAYS match on the entire X chromosome since they have one complete X in common. If the mothers are related, then some of their X will be a full match as well. So let’s look at some X chromosome comparisons at GEDmatch:
This is what the two half sisters from the question look like. The blue indicates that there is a match, the colors above it whether on one chromosome (yellow) or both (green), while red is no match. A few small errors can creep in when processing DNA so an occasional little red line can be from an error. Click the read more once you have decided if they have the same mother or the same father.
Since familytreeDNA added X chromosome matching to their family finder, all my favorite bloggers have been writing about how to use it. There is a list of their terrific articles at the end of this post.
Of course those of us who have many family members tested at 23andme have been working with the X chromosome for a while and have some thoughts on how useful it might or might not be.
- Smaller matches on the X will usually be too far back to find the relationship because the X chromosome does not get recombined when a father passes his only X chromosome on to make a daughter. Thus segments can stay together longer in time.
- A match on the X can eliminate many ancestors from an autosomal match BUT a small match on the X can be from a different line so this is not guaranteed.
- There seems to be less recombination on the X. I have heard of cases where a child got an unrecombined X from their mother. In my own family, the number of recombined segments is quite small. So it seems to me that on the female to female line the X behaves like a sluggish autosome and is not quite as actively recombined as the other chromosomes. [UPDATE 11-May-2018: the X has been shown to have a normal amount of recombination for any chromosome by a citizen science study done by Blaine Bettinger, click here for blog post by Leanne Cooper summarizing this.]
Less recombination examples
Because our maternal line (1/2 Ashkenazi, 1/2 Bavarian Catholic) is from two different population groups, my brother and I can learn which maternal grandparent we got our X DNA from just by looking at the ancestry composition for that chromosome at 23andme: