Wednesday, January 3, 2018

Desperately seeking Susan: Part 4

(Continued from Part 3)

GEDmatch one to one
autosomal and X-DNA comparison tools

Grace* and her paternal half-aunt Bridget* shared 10% more autosomal DNA on GEDmatch than AncestryDNA (737 cM versus 673 cM) as a result of different matching algorithms. They did not share any X-DNA, which is consistent with their relationship as Grace's paternal X-DNA came from her father's mother and not her father's father who was Bridget's father. (I already knew from the 23andMe chromosome browser that Bridget and Bonnie* did not share any X-DNA either.)

GEDmatch autosomal comparison
showing position only
Grace and Bonnie shared even more than that, a whopping 964 cM autosomal DNA plus a significant X-DNA match of 76 cM covering about 39% of one X chromosome. Grace did not know any sisters but was open to the possibility that one existed and Bonnie was her father's child.

Their match did not meet the matching criteria for paternal sisters however, which include sharing their father's full X chromosome (fathers only have one to pass on, while mothers pass on half of their two in a recombined mixture). Grace's father Carlo* Jr was the only child of both his Italian immigrant parents and had no full siblings, so Bonnie could not be her full paternal first cousin either. The only relationship that made sense was that Bonnie was Grace's half-niece, with a paternal half-brother of Grace's being Bonnie's father of Italian descent. Bonnie would then be Bridget's paternal half grand-niece, which also fit their shared DNA.

Not so fast.

Put simplistically, males only inherit X-DNA from their mothers and pass all of it on to their daughters only, while females inherit X-DNA from both parents and pass some of it on to all their children.** Grace’s paternal X-DNA came from her paternal grandmother and her father only passed this on to his daughters including Grace, who passed some of it on to their children. This suggested that Bonnie was the daughter of Grace’s paternal half-sister not her paternal half-brother. We can't generally tell much from the absence of an X-DNA match further out, but Bridget and Bonnie's lack of an X-DNA match was consistent with this working hypothesis.

GEDmatch X-DNA comparison showing graphics and positions

Bonnie's non-identifying information in respect of three grandparents was suddenly invalid. Bonnie's birth mother was of French, Polish, and Italian descent, while her birth father was not Italian at all (remember her ethnicity estimate did not support more than one Italian grandparent). The identity of her birth parents, including Grace's half-sister, was still unknown and we would need more clues to find them.

In JULY 2017 Bonnie announced with great excitement that her AncestryDNA results were in, and she had a direct hit. A parent match.

*This is a true story. I have permission to blog about the people who asked me for help but have not used their real names for the sake of their privacy. I have tried to limit my writing here to information pertinent to their DNA searches, but have shared other details I found with them in tree format.

**Some useful X-DNA inheritance charts have been published by Blaine Bettinger and Debbie Parker Wayne.

{Related resources can be accessed via ISOGG's Wiki page on DNA testing for adoptees.}

(Continued in Part 5)