Surprises in Genome Painting
January 20, 2025
I've largely finished getting the site working so it's pulling properly from the database, which allows me to focus more on my genome painting project. Working with my grandfather's GEDmatch kit, I've been assigning shared DNA segments to their originating ancestors. I've found the simplest method involves analyzing matches who share a single segment with him. These matches typically share about 15-30 cM and have a most recent common ancestor (MRCA) from the 18th or early 19th century.This research led to a serendipitous discovery while sorting through DNA matches on Ancestry - I recognized someone I knew, my high school Algebra teacher. This was the first time I'd ever recognized a DNA match, and coincidentally, he was also the only teacher I regularly saw outside of school. My friends and I used to show up at his house on random nights when we were in his neighborhood and wake him up. He would often hang out with us while we chain-smoked cigarettes on his porch. I even adopted a kitten from him once, and by another stroke of luck, I had recently seen him on Facebook's "People You May Know."
My teacher's match data showed that he and my grandfather share a single segment on chromosome 9, where my grandfather had no other matches. I messaged him about this coincidence and asked if he had any ideas about our shared lineage. During our conversation about his family history, he mentioned that he never knew his father and only had his name and estimated birth year. I asked him to upload his test to GEDmatch for a more detailed segment comparison. After he shared his kit number, some genealogical sleuthing revealed our shared ancestor was only one generation removed from where my previous research had ended.
The segment traced back to Isaiah VanZandt, a member of the old New York Dutch family best known as progenitors of the wealthy, ranching VanZandts of Texas (after whom a Texas county is named). Notably, this is the same family that produced the legendary folk singer Townes VanZandt, who ranks second only to Bob Dylan among my favorite artists in the genre.
Using GEDmatch's segment search, I found other matches sharing portions of this segment with both of them, allowing me to construct a cluster on this otherwise sparse chromosome on his paternal side. The fact that my teacher is a half-cousin to my grandfather makes this match particularly valuable for genetic genealogy. Usually, at this genetic distance where you share a single segment, matches trace back to an ancestral couple, making it impossible to determine which individual contributed the segment. However, with half-cousins, you can trace back to a specific individual. In this case, my grandfather descends from one of Isaiah's wives, while my teacher descends from the man's other wife, Margaret Thompson.
We were both surprised to discover such a large segment of DNA remained intact, inherited from a man born in the colonial period - more than 40 years before the American Revolution. Neat.