Deductive Chromosome Mapping: Applying it to other relatives

Recently, Blaine Bettinger, Ph.D., J.D ( gave a webinar on Deductive Chromosome Mapping.

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.

See Johnny Perl’s tips and instructions: and also a link to his new tool:

3 thoughts on “Deductive Chromosome Mapping: Applying it to other relatives

  1. Would a half sibling’s paternal (or maternal) inference be the same (though one degree closer). The portion of the segment not matching inferred to be his parent not shared with me?


    1. Not in this case. Since there is a maternal and paternal side to each chromosome. So your 1/2 sibling always matches each parent 100% at each location. Even where your 1/2 sibling matches you, the 1/2 sibling still has a non-visual match to his other parent.

      When your maternal 1/2 sibling matches you on the segment, then you know that you received the segment from the same maternal grandparent. Let say grandparent A- at this point we still do not know if it is grandmother or grandfather.

      If your maternal 1/2 sibling doesn’t match, then you know that one of you inherited grandparent A and the other inherited maternal grandparent B. Each of these non-matching segments could be different, you could have inherited the segment from grandparent B and your 1/2 sibling from grandparent A and on a different segment (even on the same chromosome) it could be vise versa. So there is not a way to designate if it is from grandfather or grandmother.

      In order to designate which maternal grandparent (grandfather or grandmother) each segment came from, you would still need cousins from each grandparent’s side to help with the triangulations.

      For example, If you and the cousin related to your maternal grandmother matches your 1/2 sibling you know that the segment is from your maternal grandmother. If your 1/2 sibling and the cousin match on a segment but you match neither on that segment, you can designate your segment as maternal grandfather and your 1/2 sibling’s segment as maternal grandmother.

      Hope this helps!


      1. Wow! Surprising and intriguing. Makes an old tired brain fire up again just to hang in. Many thanks!


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