Sharing only 360 Centimorgans with a 1st cousin

[History Lives]

Me and my known 1st cousin (my mum's brother's daughter) share only 360 Centimorgans on 23andMe. I've entered this into the Shared Centimorgan project and the histogram brings back with a more likely estimate of half 1st cousins...

If that were true, it is unbeknownst to everyone in the family and seems unlikely based on the circumstantial information we have. I don't know how any of our Shared Matches are related right now so I can't check to see if they are through our granddad or our gran to refute the possibility.

What do you think the best course of action is going forward? I believe part of the issue may be caused by her having been tested on an earlier version of 23andMe (2017) whereas I was tested on the latest version (2020), or perhaps also that she went for the Health version. My guess is this may test different SNPs and then possibly underestimate the amount of overlap between the 2 of us. Looking for any advice.

Does anyone else have any experience of unusually low shared centimorgans with a confirmed relative?

[Liz_in_Sussex]

Hi,

I don't want to get your hopes up as I think it would be important to see where your matches lead you but from my experience it is possible that the "wrong relationship" is down to the different versions of the test.

Dad and I tested with 23andme years ago and I was definitely his daughter. Mum later did a test and I was also her daughter - so no surprises - everything as expected. Dad's cousin also had the correct relationship to us both.

Then Dad and I tested with another company (I have been trying to help sort out an adoption). To my horror Dad was now my sibling. The shared cM simply were not sufficient for him to be my father. After months of the company refusing to accept there could be anything wrong on their part I was eventually offered a new (free) test for Dad at the end of last year on the grounds that his test was an old version but they would not admit that there had been any mistake in labelling us siblings. Sadly it was too late as Dad had died in the meantime. Fortunately the 23andme test (and Dad's cousin's test) supported him being my father (not to mention an incredibly strong family likeness!) but I really feel for anyone caught up in this scenario who doesn't have the proof I had because of the problems it could unnecessarily cause.

I hope you manage to sort it out but would strongly recommend following up the matches first to see if they come from just one side or both. If you have any other cousins that could test that would also be helpful.

Liz

PS I don't think it will be because of her having the Health version.

[History Lives]

Hi,

I don't want to get your hopes up as I think it would be important to see where your matches lead you but from my experience it is possible that the "wrong relationship" is down to the different versions of the test.

Dad and I tested with 23andme years ago and I was definitely his daughter. Mum later did a test and I was also her daughter - so no surprises - everything as expected. Dad's cousin also had the correct relationship to us both.

Then Dad and I tested with another company (I have been trying to help sort out an adoption). To my horror Dad was now my sibling. The shared cM simply were not sufficient for him to be my father. After months of the company refusing to accept there could be anything wrong on their part I was eventually offered a new (free) test for Dad at the end of last year on the grounds that his test was an old version but they would not admit that there had been any mistake in labelling us siblings. Sadly it was too late as Dad had died in the meantime. Fortunately the 23andme test (and Dad's cousin's test) supported him being my father (not to mention an incredibly strong family likeness!) but I really feel for anyone caught up in this scenario who doesn't have the proof I had because of the problems it could unnecessarily cause.

I hope you manage to sort it out but would strongly recommend following up the matches first to see if they come from just one side or both. If you have any other cousins that could test that would also be helpful.

Liz

PS I don't think it will be because of her having the Health version.

Hi Liz,

I'm sorry you had to go through this, though I'm somewhat reassured by what you said that there may well have been a mistake. Likewise my granddad has a resemblance to my uncle (as does my mum to my granddad). I know its not very scientific, but FamilySearch's "Compare a Face" tool has them at a resemblance of 50% !

I'm going to try and follow the shared matches, which is tricky because on 23andMe people don't tend to leave much information, and see if it leads to a connection to my granddad. If not I will see if I can get my granddad to test as he's still with us.

[Carmella]

Hi,

Have you considered uploading yours & your cousin's raw data to another DNA company for comparison?

MyHeritage and FTDNA and LivingDNA are worth looking into. Ancestry will not accept transfers in.

From the article below it appears that FTDNA will not accept certain 23andme versions - so you would have to check what versions you & your cousin have.

https://dna-explained.com/2018/09/18/myheritage-step-by-step-guide-how-to-upload-download-dna-files/

Both MyHeritage & FTDNA charge an "unlock fee" (usually approx half the price of a new test) but you may not need to pay this if you only wish to see how many centimorgans you share with each other.

Also I would say if you can test your grandad - do so anyway as it can really help sorting out DNA matches.
C

[History Lives]

Hi,

Have you considered uploading yours & your cousin's raw data to another DNA company for comparison?

MyHeritage and FTDNA and LivingDNA are worth looking into. Ancestry will not accept transfers in.

From the article below it appears that FTDNA will not accept certain 23andme versions - so you would have to check what versions you & your cousin have.

https://dna-explained.com/2018/09/18/myheritage-step-by-step-guide-how-to-upload-download-dna-files/

Both MyHeritage & FTDNA charge an "unlock fee" (usually approx half the price of a new test) but you may not need to pay this if you only wish to see how many centimorgans you share with each other.

Also I would say if you can test your grandad - do so anyway as it can really help sorting out DNA matches.
C

Hi Carmella, I had thought of doing this as I've uploaded mine to MyHeritage and FTDNA, and it was very useful, but unfortunately my cousin has since passed away so all I have to go off now is her 23andMe results.

I am trying to chase a few of our shared matches but they all seem to be inactive and or / or rather distant connections.

[JohnDoe2020]

That is a solid match at half 1st cousin. What this would indicate is that you each share only one grandparent, not a couple. This is not that uncommon, (perhaps distressing). There could be many reasons for that occurrence besides infidelity, but it does happen. I have three half 1st cousins. We all share a grandmother, but have different grandfathers. Have you tested siblings of your cousin or yourself. This can be a sticky issue...