Recently, Blaine Bettinger, Ph.D., J.D (https://thegeneticgenealogist.com) gave a webinar on Deductive Chromosome Mapping. https://news.legacyfamilytree.com/legacy_news/2020/10/advanced-dna-techniques-deductive-chromosome-mapping-free-webinar-by-blaine-bettinger-phd-jd-now-onl.html
My closest relatives and myself do not have siblings tested so visual phasing with siblings in not something I have worked with very often. After watching the webinar, I was inspired to try it with my Grandmother’s cousins.
Deductive chromosome mapping is not assigning paternal or maternal. It is zeroing in on one chromosome strand (either paternal or maternal) and assigning grandmother or grandfather to each segment on that same chromosome strand. You would not be able to use a 1C to do this because they share both grandparents. You can use any relationship as long as they are related to only one of the siblings’ grandparents and you have identified them as a maternal or paternal relationship. In Blaine’s video, he used a 1C1R (his father’s 1C).
So if the DNA match that is already identified as maternal or paternal has a segment shared by one sibling that is longer than the segment shared by the other sibling it can be inferred. The portion not matching can be assigned to the other grandparent on that chromosome. In this example: one sister matches with her maternal 2C1R “E” on the Orange segment but her sister matches “E” on the blue segment. The missing portion (yellow highlight) is the segment that can be inferred. Since it could not be from maternal grandma (or it would match the cousin) she had to have inherited it from maternal Grandpa WITT- her mother’s other chromosome. So I identified that as a WITT segment.
Now here is the neat thing for me: I can apply this segment to another relative. WITT is my brick-wall. That sister happens to match her 1C1R (cousin’s daughter) on that inferred segment-see the teal. Previously, I had it assigned more generally as a Jaroschewitz/Witt segment. Now I can assign that segment on the 1C1R profile as being a WITT segment as well!
When I went to the profile of the 1C1R, I adjusted the segment (blue) to reflect WITT, and I happened to notice that it was adjacent to a triangulation group (in green) that I suspect is related to my WITT ancestor. They all have a WITT surname in their tree as well. We just can’t quite connect our together due to lack of records. Interesting!
My Heritage has this to say about adjacent segments:
Immediately adjacent segments:
If a genetic match on the chromosome browser shows a segment that ends immediately adjacent to the beginning of another match’s segment, it is likely that they are related on the same side of your family (either both maternal or both paternal). They might even diverge at your most recent ancestral couple — one being the mother’s relative and the other the father’s relative. This is because as recombination occurs, the break in the chromosomes after they exchange information rotates between the maternal and paternal chromosome. We call this crossover point a recombination point. If you have one match who is a known match through your father’s line and another match with an unknown relationship, and the two show immediately adjacent segments in comparison to your DNA, it is likely both are related through the same line. https://education.myheritage.com/article/how-to-use-chromosome-browsers-for-genealogy/
MORE HELP NEEDED?
See Johnny Perl’s tips and instructions: https://blog.dnapainter.com/blog/more-tips-for-inferred-chromosome-mapping/ and also a link to his new tool: https://dnapainter.com/tools/isg