deciphering autosomal DNA matches

[paulzoss]

I’m trying to figure this stuff out, but I fear I have some basic misunderstandings. I hope someone will set me straight. I have an autosomal match with RL. It so happens that we both have test results on FTDNA, 23andMe, and Ancestry.com, and we match on all three, but the results are not the same. FTDNA says we are predicted to be 2nd to 4th cousins, with a 31 cM match on chromosome 8, and with total shared DNA of 62 cM across 15 DNA segments. Ancestry.com predicts us as being between 4th and 6th cousins, but says we share just 19 cM across 3 chromosomes. 23andMe says that RL and I are predicted to be 4th cousins, sharing one segment on chromosome 8 of 29cM. The only common relative we can find on our family trees (if they are correct) would put us at 8th cousins 1x removed, although there are common surnames that could, upon further study, lead to a closer common ancestor. I'm not sure where to look. My confusion is from the results of RL’s mother’s test (she was tested, but RL's father was not). According to these sites, she and I are not related. Our DNA doesn't match at all. From this I conclude that RL and I must be related through RL’s father. RL believes we must be related through his mother. I know that probabilities can work in strange ways, and that you can have a fairly close relative with whom you share little or DNA, and that through the same laws of chance, you could share quite a bit of DNA with a more distant relative, but from my understanding, if I share a long DNA segment with RL but do not share that segment with his mother, then RL and I must be related through his father. Is that right? Or am I completely missing the point? Thanks.

[The_Contemplator]

...if I share a long DNA segment with RL but do not share that segment with his mother, then RL and I must be related through his father. Is that right? Or am I completely missing the point? Thanks.

You are right, unless RL's biological mother was not tested.

The reason each company gives you different results is due to the differences in their algorithms. Some of them are going to not include smaller segments just in case they are there by coincidence which would be IBS (identical by state) instead of IBD (identical by descent). Also, all 3 test different regions of your DNA.

What you could do is have RL upload his kit and his mother's to Gedmatch.com. You do the same and you can compare them. For example you can compare RL with his mom to see if they are child/parent. As well as compare your kit with each of those.

Maybe there is something he knows that he is not making clear. If he thinks you are related through his mom, maybe his kit and his mom's are switched and he never bothered to fix it. Also just because you share no DNA with his mom does not mean you could not be related. Some names in your tree might make RL think you match him on his mom's side and RL is ignoring what the DNA tells you both.

Given how much DNA you both share, an 8th cousin once removed does not appear to be enough to cover all that. You might be related through multiple distant branches as you have probably suspected.

[paulzoss]

Thank you very much for your help.