Is it possible to fall below the lowest amount of shared cMs for a half first cousin and still be a half first cousin?

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My father's grandfather was born as one of a set of twins to a woman named Margaret, back in 1886. Margaret was a widow with other children at the time of the birth of the twins. She kept one and raised him, and gave the other (my dad's grandfather) to the other family to raise.

This is the story we've always been told but it's never been proven. My great aunt told me a bunch of details that she could remember, and I decided to research to prove it. Recently I found the paper trail and a great many details all fitting exactly with what she told me.

There is a lot of evidence pointing to Margaret as being my dad's great grandmother. I searched and found a couple of trees with her in them, contacted her direct descendants, and asked one to do a DNA test. He is also a great grandson of Margaret, and readily agreed and was very interested and happy to do so. He and my dad would be half second cousins.

The results have just come back and indeed, he is a match to my dad, but the number of cM are below the lowest number of 30 in the range of about 30-215 for half second cousins.

According to Gedmatch, my dad and the tester share two matching DNA segments, one on Chromosome 3 at 7.1 cM and 928 SNP & the other on Chromosome 11 at 8.5 cM and 806 SNPS for a total of segments > 7 cM = 15.6 cM. The paper trail is very clear and lines up exactly with what our family has always believed, and there is a cousin relationship.

Is it possible that this is just an anomaly of some kind?

EMD

Posted 2017-02-16T22:12:38.863

Reputation: 41

Why do you say he and your dad would be half second cousins? Please clarify. Did your great grandmother have children with a second man? Who is the other family? – lkessler – 2017-02-17T22:43:21.323

Yes the woman we believe to be my father's great grandmother did have children with a 2nd man. She had kids with her husband, who passed away in 1879. She had twin boys in 1886, one being my dad's grandfather who was given to another family to raise, & she kept the other one. I have pictures of her, the twin she kept, & very specific details about names she used, where she lived, etc. I found census records, death records, WWI attestation docs, all lining up exactly with details from my great aunt. DNA test of her known great grandson confirmed a relationship to my dad but < than 1/2 2nd. – EMD – 2017-02-18T23:33:17.180

According to the chart I used to determine possible relationships based on cM, 1/2 2nd cousin could fall anywhere between 30-215 cM. My dad and the tester share 15.6 cM. Is it still possible that they are 1/2 2nd cousins? We are not aware of any sisters of this woman who may have been the actual mother of the twins, but who knows, there may have been. I haven't found any in the official records in Grey County, Ontario where they were living. I can't see how my dad and the tester could show a relationship (and have shared matches) if they aren't related in some way. – EMD – 2017-02-18T23:41:30.303

Also, the very unusual details I have about this woman and her life, and the fact that we have a picture of her and of the twin she kept (my dad's grandfather's full brother), it all just seems too much to be coincidence. – EMD – 2017-02-18T23:43:02.887

Thank you so much, lkessler, for taking the time to help me with this mystery. I appreciate it very much. – EMD – 2017-02-18T23:44:29.180

Answers

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Your paper evidence seems to be fairly conclusive that the two are half second cousins.

The discrepancy to me appears to be your summation of the GEDmatch matches. It shows two segments over 7 cM that total 15.6 cM. Those are the two matching segments a GEDmatch One to One would show since by default it doesn't show segments smaller than 7 cM.

You are comparing that to the half second cousin expected range of 30-215 cM. I think if you check those tables, you'll find they are made up of the total of all matches greater than 1 cM.

Try the GEDmatch One to Many comparison. The total cM from that includes the small segments. That number should fall in the 30-215 cM range.

lkessler

Posted 2017-02-16T22:12:38.863

Reputation: 16 148

Thank you once again, for your response to my query. You have been a geat help. Erin – EMD – 2017-02-19T15:24:15.640

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Yes, it is possible. Because the inheritance from our grandparents is statistical matter and we don't bear precisely 25% of DNA from each grandparent. This number can be almost any in wide range. The idea behind it is greatly explained in next article: https://gcbias.org/2013/10/20/how-much-of-your-genome-do-you-inherit-from-a-particular-grandparent/

So there is no strict boundaries of shared cMs between half cousins.

George Gaál

Posted 2017-02-16T22:12:38.863

Reputation: 1 075

Thank you so much for your input. I greatly appreciate this. Erin – EMD – 2017-02-19T15:21:55.267

No. Your answer is not true. Statistically, each chromosome has an expected number of crossovers that approximate a Poisson distribution and due to the statistical law of large numbers, 46 distributions added together will tend towards the mean with a calculable standard error. The ranges that Blaine Bettinger found in his sample that were quoted in the question illustrates that there is an expected range that is not limitless. – lkessler – 2017-02-22T04:27:41.323