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How Much DNA Do We Share? It Depends

Roberta Estes

I was curious how testing the same two people at the 3 different vendors, then uploading the results from those different vendors to GedMatch and repeating the matching process there would affect the amount of DNA reported as matching.

I have a third cousin who has tested at all 3 labs independently, meaning they did not upload a file from either 23andMe or Ancestry to Family Tree DNA. Furthermore, they downloaded their 23andMe and Family Tree DNA files to GedMatch. They have not downloaded their Ancestry results to GedMatch, so I can’t do the Ancestry to Ancestry comparison, unfortunately.

So, we have one pair of third cousins, 3 individual vendor tests (each) and 8 independent answers to the question, “How much DNA do we share?”.

First, the theoretical expected average (as reported on the ISOGG wiki page) is 53 cM for third cousins. Blaine Bettinger’s actual findings through the shared cM project indicate an average of 79 cM for third cousins, and the actual range found is 0-198 cM, after removing outliers. This isn’t the first time in genetic genealogy that we’ve found that the theoretical or expected results aren’t what really happens as we learn more about how DNA actually works.

Let’s see how reality stacks up for our third cousin pair.

Vendor Threshold Total cM Total Segments Largest Segment Est Relationship
Theoretical 3C Average, Actual Average and Actual Range 53 ISOGG, 79 Actual, Range(0-198)
At Vendors
FTDNA 7cM/500 SNPs 149*** 22 33.52 2nd-3rd cousin
23andMe 7cM/700 SNPs 134 6 40.8 2nd-3rd cousin
Ancestry V1 5cM after Timber** 132 8 Not provided 3rd-4th cousin
At GedMatch
GedMatch 1* (23andMe V3 to 23andMe V3) 7cM/700 SNP 147 6 43.7 3.3 gen to MRCA****
GedMatch 2* (FTDNA to FTDNA) 7cM/700 SNP 136 6 43.7 3.4 gen to MRCA****
GedMatch 3* (23andMe V3 to FTDNA) 7cM/700 SNP 136 6 43.7 3.4 gen to MRCA****
GedMatch 4* (Ancestry V1 to 23andMe V3) 7cM/700 SNPs 147.5 6 43.7 3.3 gen to MRCA****
GedMatch 5* (Ancestry V1 to FTDNA) 7cM/700 SNPs 147.5 6 43.7 3.3 gen to MRCA****

Total cM is rounded except for 147.5, which doesn’t round in either direction.

*GedMatch at default setting which is currently 7cM and 700 SNPs.

**Unknown if SNPs are being utilized at Ancestry as a threshold parameter, and if so, the threshold is unknown.

***Total cM at Family Tree DNA includes small segments if you match. At 23andMe and GedMatch, total segments means only the total number of segments over the match threshold. The number at Family Tree DNA would be 112 cM if only counting segments greater than 5cM and 107 if only counting cM greater than 7. Of note, in my comparison, there no matching segments between 5.48 and 11.09, so this may be an unusual circumstance.

****The actual generations to a common recent ancestor (MRCA) is 4, counting our parents as generation 1.  It is unclear whether GedMatch counts you as generation 1 or your parents as generation 1.

Results like this are a perfect illustration of why relationship ranges based on DNA are ranges, not absolutes. I know, unquestionably that my cousin is my third cousin. However, were I to utilize ONLY the averages, I would be looking at either a 2nd cousin utilizing the theoretical numbers or a 2nd cousin once removed utilizing the real average, neither of which are accurate in this case.  Averages are made up of everyone in the range, smallest to largest – and in this case, the results fall into the larger than average category.

All of the Total cM numbers are two to three times the theoretical expected Total cM, but all of the Total cMs are still within the observed and reported range for third cousins.

For more on relationship ranges, theoretical expected versus actual and ranges as reported from crowd sourced information see here and here and here.

Blaine Bettinger provides a free download of his latest Shared cM Project results, which includes a great chart on the last page that provides a minimum, average and max cM shown for each relationship type. Thanks Blaine, for this very useful tool!

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