Total Shared cMs Are Important, A Chromosome Browser Is More Important

 

I  asked Gemini AI which technique of evaluating DNA matches is the best? Is total cMs shared or comparing segment data better? I agree with this chatbot that if you are serious about using DNA for genealogy both segment data and a chromosome browser are best

It is true that total cMs can be helpful when it comes to identifying close family. The new Ancestry Pro Tools Advanced Matching allows for comparison of cMs with shared matches, and matches you share with them. It also shows relationship predictions of your matches shared matches. It can also spot NPEs when total cMs are very different from what is expected. 

Total cMs isn't as helpful at all for matches beyond a couple generations. As you see below total shared cMs can vary quite a bit between first cousins, and matches we share. I share 173 cMs with a 2nd cousin 1x removed. My cousin Judy shares 248 cMs, and my cousin Darryl shares 350 cMs. My cousin Darryl shares 177 cMs more than I do with our 2nd cousin 1x removed. 

Generally I share around 1000 cMs with my first cousins. Oddly my mother shared fewer cMs with a couple 1st cousins descended from the same uncle. One first cousin shares only 534 cMs with my mother. I thought possibly there was an NPE. No, this cousin has matches on the correct lines. Another cousin, who is a half first cousin, only matches on our maternal grandmother's side and shares around 500 cMs also. Lower than expected matches need further evaluation. 

This match's brother shared only 782 cMs. Both of these cousins tested at AncestryDNA. I wonder if Timber chopped down some segments leading to these lower cM shared totals? 

My mother only shares 13 cMs with a distant cousin, but a first cousin shares 32 cMs. This demonstrates the wide range of cM totals, and the difficulty of using autosomal DNA to predict relationships. Different relatives may share more or less DNA with the same distant cousin.  

With such a wide range of relationship possibilities total shared cMs aren't that helpful. Building a chromosome map with segment data you can associate certain segments with particular families. Just looking at cMs can be helpful for close relationships, but can be a waste of time for distant relationships. 

A chromosome browser is definitely essential when trying to confirm distant matches, and solve a brickwall. I've been going through my chromosome map analyzing segment matches, and making some corrections. I can't extend it much farther at this time because fewer people test at MyHeritage and Family Tree DNA. 23andMe no longer allows you to download shared match segment data. It's difficult to get matches to upload to GEDmatch. Many just don't understand how, or what information it would provide. 

I've marked my segments, from my Genome Mate Pro segment map, with names of the ancestors who contributed them. I'm not able to visually phase my segments to recreate the segments passed down to me by my grandparents because you need siblings to do that. I'm an only child. I marked the segments I received from each grandparent using Microsoft Paint. I was thinking of using DNA Painter, but that would have taken too long. I would have had to upload segments, and mark them from scratch. 

Since AncestryDNA is unlikely to ever provide segment data my hope is 23andMe allows this data to be viewed and downloaded again, and the other companies attract more testers. Otherwise, I can't make any more progress using DNA.