August 2017 Update to the Shared cM Project

The Shared cM Project is a collaborative data collection and analysis project created to understand the ranges of shared centiMorgans associated with various known relationships. As of August 2017, total shared cM data for more than 25,000 known relationships has been provided. To add your data, the Submission Portal is HERE. I am always collecting data, and perhaps the next update with have 50,000 or 100,000 relationships!

This August 2017 update is the second update to the original data, released in May 2015, and includes many thousands of new submissions.

There is MUCH more about the project, including histograms and company breakdowns in the PDF download.

Figure 1. The Relationship Chart

Table 1. The Cluster Chart

Sample Histogram from the Shared cM Project (all histograms available in the PDF download):

Sample of Table 3. Analysis of endogamy and company breakdown for 1C (all company breakdowns available in the PDF download):

Sample from Table 4:







18 Responses

  1. Maureen 27 August 2017 / 10:37 am

    Wow! What a tremendous amount of work. Thank you, Blaine! I used your charts all the time.

    I have a lot of known cousins now whom I want to enter as data points into your project. But…I do not remember exactly whom I have already entered (very few I think). Will you be able to weed out duplicate entries (maybe based on my email address)? I will keep track now of whom I enter…

  2. Jim Davis 27 August 2017 / 1:58 pm

    An invaluable resource, THANK YOU!

    I haven’t read the pdf yet, but is there not enough significant difference between testing company results to warrant additional charts?

    • Jim Davis 27 August 2017 / 2:01 pm

      Ignore my question (not my thanks though). Some day I’ll learn to read a bit more before I post, lol.

  3. Curtis Rogers 27 August 2017 / 9:41 pm

    I frequently refer your chart to users of GEDmatch. This update will be a big help. It is an invaluable aid to the genealogical community and an instant classic tool .

  4. Liane Jensen 31 August 2017 / 9:14 pm

    Thank you so much for the time and effort you spend generating these references for the community.

  5. Kathy Rooney 1 September 2017 / 5:38 pm

    I have put your chart on my phone! Thank you. I have one question about the average for the 4c1r, is it supposed to be 29? That makes it higher than the 4c1r. Thank you!

  6. Rebecca 3 October 2017 / 12:32 pm

    We reported a 480 from FT for cluster #3 – which is literally off your chart! (We have no reason to suspect anything non-parental.) Gedmatch gives the same pair a sightly higher 541cM.

    aunt-nephew M-J 1,552 longest block 140 (gedmatch:1685.8 133.9) cluster 2
    father-son J-D 3,383 longest block 267 (gedmatch:3586.5 281.5) cluster 1
    greataunt-greatnephew M-D 480 longest block 72 (gedmatch:541.2 71.6) cluster 3

  7. Brad Hurley 6 October 2017 / 4:55 pm

    As you noted in your PDF, different testing companies calculate the total shared cM differently, depending on the minimum size segments that they include in that total. What minimum size did YOU use when creating this chart?

    Thank you very much for this extremely valuable resource.

    • Kathleen Hurley Doan 10 November 2017 / 3:36 pm

      Are you on Kathleen HURLEY Doan

  8. Judy Palmquist 13 October 2017 / 11:58 am

    Love your book The Family Tree Guide to DNA testing an Genetic Genealogy. I have entered several people I’m not sure if I entered all of them. When you get 50% of Father and 50% of Mother’s dna this is just an average right. How much can this vary? In the book between grandparent and 2 grandson’s the difference 22-28 for 1 and 17.7-32.3 for the other. Judy

  9. Sue Lambert 22 October 2017 / 7:17 am

    I don’t see any replies on here from the author but hopefully they are reading. I didn’t see any spot on the interactive chart for identical twin result. Identical twin is confusing as it appears to be the same cM’s as parent/child. You would think it would be much more. An identical twin article would be an interesting read for me and maybe others

  10. Linda R Horton 27 October 2017 / 9:56 pm

    Dear Blaine, thanks again for the great work. I am presently investigating the match to me, on AncestryDNA, of a half nephew (son of a half sister). Am I correct in seeing minor differences in the numbers on the Relationship Chart vs. the Cluster Chart? The Relationship Chart states that, for the relationship of half nephew, the average amount of shared cM was 891 and the range was from 500-1446 cM. relationship chart was 891 and the range was from 500-1446 cM. The cluster chart says for cluster 3 relationships the average is 884 and the range 619-1159. Am I missing something? Thank you

  11. Tom Ragusin 5 November 2017 / 4:45 pm

    Great work with the shared cm segments because it is essential. We have a very valuable tool (autosomal dna testing), but the statistical nature of the results make it nearly unusable except for the closest matches. An accurate, well prepared genealogy is absolutely necessary to make sense of the results for more distant matches. The hope is that your work will refine the statistics for more distant matches. However, there are several assumptions that must be stated.
    The first assumption is that the genealogies used to determine our degree of relationship are accurate and well prepared. I will not use family trees from some genealogical websites because I am not always able to confirm the provided information. This problem becomes quite important when attempting to differentiate at some levels of relationship (for example 8th cousins and 8th cousins once removed).
    The second assumption is related to the first. For the statistics to be valid the two related persons must be related in only one way. This becomes less likely at more distant generations but is harder to prove. This would require knowing all 512 persons in our 10th generation (8th cousins) to provide valid information for your tables.
    The third assumption is that we all understand endogamy the same way. Incest and marriage not permitted by a church are obvious, but what of 2nd or 3rd cousin marriages? Second and third cousin marriages have a measurable and not insignificant impact on the collected evidence. Similarly, fourth and fifth cousin marriages have a measurable, but smaller, impact on the statics. Unfortunately, the last two examples are harder to prove without an extensive and accurate family tree. This is extremely important at the most distant relationships where the uncertainty, as currently reported, far exceeds the measured or predicted value.
    The assumptions mentioned must be evaluated for every set of data submitted.

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