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Mike_G
07-31-2017, 05:48 PM
I have a couple cousin matches who are brother and sister. My shared DNA with her is 254 CMs across 12 segments. I share 182 across 12 with her brother. Is is normal to have such a large discrepancy like that? Thanks.

spruithean
07-31-2017, 05:57 PM
I think it's within range for 2nd cousins and 2nd cousins once removed. If I remember correctly. What degree of cousin are these cousins?

Perhaps his sister and you inherited more of the same genetic material from your shared ancestor than her brother did?

C J Wyatt III
07-31-2017, 06:04 PM
I have a couple cousin matches who are brother and sister. My shared DNA with her is 254 CMs across 12 segments. I share 182 across 12 with her brother. Is is normal to have such a large discrepancy like that? Thanks.

Are all of the kits on GEDmatch (and are the same type from the same vendor)? I am not looking for numbers, but I have an idea about something you might try.

Jack Wyatt

Mike_G
07-31-2017, 06:05 PM
We're half first cousins, once removed (or is that half second once removed? That stuff always trips me up) -- their grandmother, my great grandmother but different fathers.

Mike_G
07-31-2017, 06:10 PM
Are all of the kits on GEDmatch (and are the same type from the same vendor)? I am not looking for numbers, but I have an idea about something you might try.

Jack Wyatt

No Jack, they're not on Gedmatch. I'm in continuous contact with them, so if it's something they might be interested in looking into, I'll recommend it. His results just came in today. They're super nice people, but they haven't been pursuing the DNA questions with the uh....vigor that most of us on AG display.

geebee
07-31-2017, 06:10 PM
I have a couple cousin matches who are brother and sister. My shared DNA with her is 254 CMs across 12 segments. I share 182 across 12 with her brother. Is is normal to have such a large discrepancy like that? Thanks.

It isn't unusual, anyway. I'm guessing that Ancestry predicts the sister to be your 2nd cousin, and that the prediction for her brother is perhaps 3rd cousin. Obviously, whatever one of them is, the other is the same.

The average sharing between 2nd cousins in The Shared cM Project (http://thegeneticgenealogist.com/wp-content/uploads/2016/06/Shared-cM-Project-Version-2.pdf) was 238 cM, and for 3rd cousins it was 79 cM. But these were just the averages -- the range for 2nd cousins was from 42 cM to 504 cM. So you can see that both siblings fall within the range.

The range for 3rd cousins was from 0 to 198 cM, and even though the brother does fall within this range, his sister does not. So I'd say it's unlikely that they're 3rd cousins. However, the range for 2nd cousins once removed was from 0-325 cM, and both siblings are within this range also. So it's likely that the siblings are either 2nd cousins or 2nd cousins once removed.

I have a 2nd cousin at 23andMe with whom I share 207 cM in 11 segments. But one of my siblings shares 272 cM in 12 segments with this same cousin, while another shares only 161 cM in 10 segments. The average sharing for all six siblings with this second cousin is 215 cM.