If you’ve been studying genetic genealogy for a bit, you may understand X-DNA, but I’ll give a summation here.
X-DNA is different from regular autosomal DNA. The 23rd chromosomal pair contains the sex chromosomes – those which determine the biological gender of the child. In all but rare cases, biological females have two X chromosomes (XX), and biological males have one X and one Y (XY). As we may have learned in high school or college biology, it is the male’s 23rd chromosome that determines the gender of the child.
Therefore, if the offspring is biologically male, the father’s Y-DNA has been combined with X-DNA from the mother to create an XY 23rd chromosomal pair. If the offspring is biologically female, the father’s X-DNA has been combined with X-DNA from the mother to create an XX 23rd chromosomal pair. Thus, a male’s X-DNA cannot be passed to any of his male offspring.
So, let’s see how this works.
K and I are an autosomal and X-match. Our trees show that we are 4th cousins. K is female and so am I. We knew already that our most recent common ancestors are Absalom RENFRO (1810-1855) and Elizabeth CORMACK (1809-1901), so this one was pretty easy and is a great illustration of how X-matching works.
K and I match autosomally on Chromosome 15, shown here from my data on DNA Painter.
Absalom and Elizabeth were the parents of 12 biological children and one adopted son (who I suspect was their orphaned grandson). I descend from their son John Marion RENFRO, and K descends from their daughter Mary Esther (RENFRO) SPAULDING.
Because Mary Esther was a daughter, not a son, of Absalom and Elizabeth, she inherited X chromosome DNA from each of her parents. However, because John Marion was a son, he inherited one X, only from Elizabeth.
This means that our X-match cannot be from Absalom – it must be from Elizabeth, and I could not have inherited any X-DNA from Absalom.
So here it is on DNA Painter, on the blue (paternal) half of Chromosome X because the RENFRO/CORMACK line is on my paternal side, and labeled with the proper ancestor:
I love the size of this X-match.
Thus, anyone on my paternal side with whom I have an autosomal and X-match, and the matching DNA overlaps the segment on the X where K and I match should be descended from either Elizabeth CORMACK, her parents, or any of her ancestors who could have passed down to her an X-chromosome through her parents, unless the X-match is too small to be valid. Note that the autosomal matching segments do not need to be on Chromosome 15. This is simply where K and I match.
My Chromosome 15 match with K could be with any of our common ancestors. It doesn’t have to be with our X-match Elizabeth CORMACK. Ideally, we would be able to triangulate with other descendants of Absalom and Elizabeth so as to verify that our relationship is through that line and that the X-match does not come from someone else. It gets tricky with pedigree collapses, and you don’t know what you don’t know until you know it.
Below are two fan charts showing the ancestries of K and me relevant to this discussion. I have put a box around our most recent common ancestors. This chart shows the X inheritance paths as they relate to the newest person in each chart, i.e., K or me. Note that we are at the bottom. Each person who could have passed an X down to get to either of us through this RENFRO/CORMACK line is marked with a red X. You can see that, for example, although K could have, and may have, received X-DNA from Absalom RENFRO, I could not have, so we can’t match with that ancestor on the X.
In these images, we can visualize what I said earlier about from whom the X-match has to have come.
I hope this has been helpful. Please comment below. Thank you for reading.
- Phasing the X-Chromosome, Jim Owston, The Lineal Arboretum (http://linealarboretum.blogspot.com)
- X Marks the Spot, Roberta Estes, DNA Explained (dna-explained.com)
- That Unruly X….Chromosome That Is, Roberta Estes, DNA Explained (dna-explained.com)
- X Matching and Mitochondrial DNA is Not the Same Thing, Roberta Estes, DNA Explained (dna-explained.com)
- Roberta Estes’ series on DNA Painter
- X-chromosome testing, International Society of Genetic Genealogy Wiki (isogg.org/wiki/)