A Segment-ology TIDBIT
Normally your child’s atDNA cannot indicate which side of your DNA a Match is on. But, a child only gets one side of your DNA at a time. In other words: you have DNA from both of your parents that covers each of your chromosomes from beginning to end; but you can only pass on one side to a child. Your parents are your child’s grandparents – every bit of your DNA passed to a child is either one of the child’s grandparents (your parents) or the other. Please review Bottom-Up and Crossovers to see the approximate 34 crossover points that divide all of your child’s DNA into about 57 segments from your parents. This is a very important concept in atDNA, and well worth the time to study it until you understand it.
So if you have a segment that you cannot figure out which side it’s on (your maternal or paternal side), having a child also match on that segment doesn’t give you any insight. Because: if you don’t know which side it’s on for you, you don’t know which grandparent it’s on for your child.
However, if you have tested at multiple companies, a child may help. Here is an example.
At 23andMe I have 3 Matches who all match me and each other on the same segment – a Triangulated Group (TG). There are no other 23andMe Matches, or any known cousins, who overlap this segment at 23andMe. However at GEDmatch I have a number of Matches with overlapping segments, and I’ve formed two TGs. And with my Dad’s kit at GEDmatch, I know which TG is paternal and which is maternal. I also have a child at GEDmatch, and he matches the maternal TG, but not the paternal TG. So my son got my maternal DNA for this segment. This son was also tested at 23andMe, so checking the ICW list with the 3 Matches above, they all include my son with a “Yes” – meaning an overlapping segment. So I can conclude that the 3 Matches at 23andMe form a maternal TG for me, because my son only got my mother’s DNA on this segment.
This now lets me communicate this information to each of the 3 Matches: our Common Ancestor will be on my mother’s side (and I include a list of Patriarchs of those lines in my message). I’ve made up two standard messages for 23andMe Matches – one for each side – which I can quickly copy and paste into the 23andMe message box. I initially get about 10-20% response – usually with a thank you for providing a concise, and easy to review list of potential Common Ancestors. Over time, additional responses trickle in. It’s a quick and easy way to find those Matches you can work with. And sometimes these “seeds” bear fruit months and even years later.
[22L] Segment-ology: Using a Child to Determine the Side TIDBIT; by Jim Bartlett 20170712