On average, how would range of shared centimorgans compare between first cousins who have same paternal grandmother versus first cousins with different paternal grandmothers, same paternal grandfather in both cases.


1 Answer 1


Another way you may see this relationship described is "half first cousin", meaning the cousins only share one grandparent.

The Shared cM Project is my go-to source for such information. According to their most recent data the expected range is between 236 and 704 cM, with an average of 554 cM. Note that this range is based on a small sample size of 23. It would therefore not be surprising if a small number of half first cousins fell outside this range. You nevertheless can use this range as an estimate of how much DNA half first cousins will typically share.

  • The same survey data study shows that full first cousins with a range of 533 to 1379 cM, averaging 880 cM, so the lower part of this range overlaps with the upper part of the half-first-cousins range.
    – RobertShaw
    Apr 7, 2017 at 3:36
  • @RobertShaw Yes, due to the randomness of genetic inheritance there are very few DNA results that are definitive for a specific relationship. In these cases you have to use DNA in combination with traditional genealogy methods to come to any conclusions about relationships.
    – Harry V.
    Apr 7, 2017 at 10:37
  • The second link in this answer is broken.
    – PolyGeo
    Sep 19, 2018 at 23:57
  • @PolyGeo Both links are still working for me.
    – Harry V.
    Sep 20, 2018 at 3:42
  • @HarryVervet - me too ... now! Not sure what was happening earlier.
    – PolyGeo
    Sep 20, 2018 at 3:46

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