High shared DNA % for siblings

Question

I am helping someone with their DNA.  A brother and sister both tested with 23andMe.  23andMe is showing that they share 59.4% DNA.  One of them contacted me and asked if I thought this could be correct.  I was allowed access to the data.  I downloaded the aggregate data to see how many cMs they share and then added the shared cM column, resulting in a total of 4,416.7 cM.

This seems higher than acceptable.  Blaine Bettinger estimates siblings should share 2,209-3,384cM.  She has asked 23andMe to redo her test as she thinks it is wrong.  I would like to know what our DNA group on here thinks about it . . .

Answer 1

I think there's a possibility that the results are accurate...due to the way 23andMe performs their computations. I would expect quite different numbers if both kits were uploaded to GEDmatch and compared there.

At 23andMe, where there is at least one shared segment, they count the total, combined length of all the half-identical regions. This is what the other companies do; but since a portion of the child's genome is expected to be fully-identical, where there are two shared segments at the same location, 23andMe extends that and double-counts that fully-identical segment. Also, 23andMe adds any X-chromosome matching segment(s) into the total, which the other companies, and GEDmatch, don't do.

For comparison, FTDNA considers each gamete as contributing about 3400cM of material, for a total of about 6800cM; autosomal, no X counted. GEDmatch uses the same basic assumption, to the best of my knowledge. At 23andMe, though--and with the X included--a female genome is considered to be 7439cM, and a male's 7257cM...the difference there being the X-chromosome. The 4,416.7cM amount still seems high, but I can't speak to way 23andMe actually presents the raw data. If you're totaling using something like MS Access or Excel, we may be double-adding some values, like the fully-identical segments in the autosomes.

If uploaded to GEDmatch, I'd think the match should be more in-line with what you'd normally be expecting. I can't say with certainty this is the case, but if they're willing to create GEDmatch accounts it will be a quick way to check.

Comments

Thank you!  I was hoping you would pipe in on the matter!  (I almost sent you a PM instead of posting on G2G.)  I tried to talk the person into uploading to Gedmatch, but she is concerned about privacy issues.  So...

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And I kept an answer under 300 words! Wow! I must be tired tonight...

Thanks for the best answer star, but I'm honestly not 100% confident I'm correct. Maybe 90% confident that's not an impossible explanation? I don't have two full-siblings' raw data from 23andMe I manage that I can check for you at GEDmatch. Maybe someone else will. Luck!

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I can illustrate Edison's answer with my data from 23andMe and Gedmatch.

My sister and I both tested on 23andMe, which says we are share 48.2%. 23andMe reports that we have half matches on 2653 cM and full matches on 931 cM, for a total of 3584 cM (which is 48.2% of 7439). So, as Edison stated, 23andMe reports a percentage based on double-counting the segments where we got DNA from both parents. (And this includes the X chromosome, where 23andMe counts a total length of 182 cM and adds on another 106.5 cM for the parts where we have a full-IBD match.) And although our match is less than 50% of our DNA, they calculated that with a cM statistic that's higher than the high end of Blaine Bettinger's range for siblings.

Now to Gedmatch. Gedmatch says my sister and I match on a total of 2522 cM (toward the low end of Blaine Bettinger's range for siblings) on chromosomes 1-22. And it shows us with a match of 195 cM (the full length, apparently calculated differently than 23andMe) on the X chromosome.

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If you go into 23andMe's DNA Comparison data page, take the total length of half-IBD segments that's reported at the top of the page, then subtract from that value the length of the half-IBD segment(s) for the X chromosome, you should get a value that is close to what Gedmatch would report. (When I do that arithmetic, I get 2471 cM, which I think is reasonably close to the 2522 cM I see on the Gedmatch one-to-many comparison page and the 2509 cM is see on the Gedmatch one-to-one comparison.)

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Ellen, thanks.  That's a big help.  23andMe shows them sharing 3052 cM half-identical segments, then looking at the X chromosome it's at 61.2 cM, so takes them to 2990.8 cM.  That brings it within the range of what's to be expected.

Answer 2

It does seem high, but nothing is impossible. Have you checked the "Are your parents related" on GEDmatch?

I presume they were born from separate pregnancies

Tim

Comments

Parents are not related. Yes, separate pregnancies - born two years apart.