View Full Version : An x matrix for my father

03-31-2020, 04:53 PM

Worth looking into or not?

03-31-2020, 09:26 PM
Those are centimorgans?

03-31-2020, 09:42 PM
Total cMs .

04-02-2020, 09:48 PM

Worth looking into or not?

I would think so. Some of those segments are pretty large -- in fact, one of them is pretty close to being the full length of the chromosome. The most I share with anyone on the X chromosome (outside of close family members is) is 66.4 cM at GEDmatch. That's in two segments, one of 25.8 cM and one of 42.4 cM.

This person is a 3rd cousin twice removed, but it's actually somewhat more complicated than that. His parents were 1st cousins, and I'm related through both of them. But obviously he received the shared segments on the X chromosome only through his mother, just as I received them from my mother.

I've been able to backtrack the possible contributors on each side, and our most recent common X-chromosome ancestor is my 4th great grandmother and my cousin's 3rd great grandmother on his mother's side. (The same ancestor is also his 2nd great grandmother on his father's side.)

What this means is that we're actually 4th cousins once removed as far as our shared segments on the X chromosome are concerned, even though we're a bit more closely related from a genealogical standpoint. (My genealogical software actually lists five different relationships, but only one of them contributed to our shared X-chromosome segments. Any of them could have been responsible for our shared autosomal segments, although on my side there's only one path.)

I suspect your father's relationship to any of these matches is not likely to be this complicated, but keep in mind that the actual number of X chromosome contributors out there is going to be much less than for autosomes. Obviously, fathers don't pass an X chromosome to their sons; and mothers don't pass on anything from their paternal grandfather's X chromosome.

Think of it this way. Without pedigree collapse, your number of autosomal ancestors doubles as you go back through each generation. So by the time you get to 4th great grandparents, you have sixty-four of them. The increase in possible X-chromosome ancestors, however, follows the Fibonacci sequence. For males, begin with 1, then 2, 3, 5, 8, 13, 21, 34, etc.

This is the maximum number in each generation. It might be even fewer in reality. For example, my eldest brother's X chromosome is only from our mother's father, with no DNA from our mother's mother. So we can actually begin the sequence with our grandfather. This means that, for my brother, only one great grandmother contributed to his X chromosome, instead of the three possible. So by the time you get to 4th great grandparents, there are only five possible contributors instead of thirteen.

If you compare this to the autosomes, that's five versus sixty-four. So not only can it be worthwhile to pursue matches on the X chromosome, sometimes it can even be easier because of the smaller number of possible contributors on each side.

(I might note that on my cousin's side, the "X-path" from our most recent common X-segment contributor involves the fewest possible crossovers. The path is from our shared ancestor, Rachael, to her son Willis, to his daughter Rachel, to her son Felix, to his daughter Dorothy, to my cousin. On my side, the path is from Rachael to her son John to his daughter "Polly" to her daughter Hannah to her son (my grandfather) to my mother to me.)

06-29-2020, 11:36 PM
Matrix with matches of 20+


12-13-2020, 07:11 PM
Matrix with matches of 20+


Very useful, thank you.

01-11-2021, 09:46 PM

Worth looking into or not?

If these are all on the X, you have wonderful material to work with.
Hopefully you also have some good guides - some mentioned in threads here - such as:
Advanced Genetic Genealogy ed Debbie Parker Wayne, Ch3, or
something from blogs, say https://dna-explained.com/
Good luck