Relatives Sharing a Segment on the X Chromosome

This is a group of relatives who share segments of the X chromosome from at least one common ancestor. Because of the special status of the X chromosome, all the relatives listed here are kin of my mother's side of the genealogical tree. The X chromosome is a sex determining chromosome: an XY defines a male, and an XX a female. The X chr. can be passed down from male to female, but not from male to male. All men, therefore, get their X chr. from their mothers. Consequently, the genealogical tree that reflects the passing down of the X chr. cannot have two successive generations of males on it. So, in traveling up the genealogical tree, the branch belonging to the father of any male on it can be pruned. The tree thus trimmed down consists of all those ancestors who are candidates for having passed down the X chr. to the base person on the tree. Such a tree of potential sources for these X chr. correspondences can be seen at Possible Ancestral X Chromosome Sources (.ged file). For people related to me as 4th cousins, this set of possible X chr. ancestral sources would be: Nancy Parks (b. 1824), or Mary Beaty (b. 1801), or Robert or Ruth Buckhannon (b. ca. 1790s), or Nancy Lennard Benson (b. 1781), or John Franklin Harbour (b. 1780), or Jane Moore (b. 1803). These are all British names (English, Scottish, Welsh), and as predicted by 23andMe with about 80% confidence, my entire X chromosome is solely "British & Irish." Another X chromosome fragment, a very short one beginning at position 1, came down from George Washington White, who obtained it from his mother Nancy Benson (see the study of chromosome 5). This segment is completely unrelated to the material under review here. So the massive X chromosome material here had to have passed through Geo. W. White's first wife, Mary A. H. Harbour, who acquired it from her mother, Jane Moore, or from the mother of J. Franklin Harbour, Judith Strange. So this narrows it down significantly.

At least superficially, there appear to be two distinct segments, and perhaps three. However, in every case (except Shirley Daniels), the first segment is correlated with the occurrence of the second segment, which shows that in some way they belong together. The second and third segments overlap in three cases (Mueller, Shafer, and Arnold). For the following people the first segment begins precisely at 46163271: Grubbs, Crumby, and Donovan; and for a surprising number of people (Beckmann, James, Rocknic, Helm, Crumby, Donovan, Willis, Grubbs, Sanchez) the first segment also ends at precisely the same spot: 87774007. The second segment begins at 110500583 for a large number of people: Willis, Grubbs, Helm, Mueller, Donovan, James, Rocknic, and Franklin; for Rocknic, Helm, Langford, James, Willis, Brown, and Moss. For this group it ends precisely at 116769516; except for Diane Roberts and Ashley Dunn, where it ends at 117314383. The third segment begins at 129579884 for Roberts, Helm, Dunn, Sanchez, Moss. Such a degree of correspondence has not been seen in any other chromosome thus far studied.

Another strange feature of this X chromosome group is that everyone who has segment 1 also has segment 2, but not vice-versa. Segment 1 and segment 2 seem to have originally been passed down together, but segment 1 dropped out at some point in history for one or more lines of descent.