Sunday, August 21, 2011

Known Relative Studies with 23andMe: Second Cousins or Half Second Cousins? (2nd Cousins, Part Three)

In the first two installments of my second cousin studies, I have focused on my maternal side. This time I am focusing on a second cousin from my paternal side. Our shared ancestors are our great grandparents Willard Moore (1877-1934) and Blanche (Purdy) Moore (1888-1935) from Washington State. I am descended from their first son Fred and this cousin is descended from their second son Jack. I don't have as many family members tested on that side, so I have less comparisons to share, however this study brings up some interesting questions.

Fortunately, I have both of my sisters tested at 23andMe. The amount of shared DNA between us and our paternal cousin (PCousin) are 1.15%, 1.87% and 2.4% (in order of our testing). Since the expected percentage of DNA sharing is 3.125% on average with a second cousin and 1.563% with a half second cousin, I was beginning to wonder if we were really full second cousins, sharing both of our great grandparents. However, as my sisters' results came in the percentages were higher and higher. I think it is very interesting that this second cousin shares twice as much with the third sister tested as the first. The percentages are still a bit lower than expected, so more comparisons are needed to determine the exact relationship.

Notice also our paternal cousin shares on only two small places with all three of us. The other blocks of shared DNA are all randomly distributed.

Click to enlarge chart

Since I have tested both of my father's siblings, I am also able to compare them to this cousin. Their relationship is first cousins once removed with an expected percentage of DNA sharing of 6.25% on average. When my uncle's results came in, the match was only 3.33%, again consistent with a half cousin relationship. Then, against the trend, my aunt's results showed sharing of 6.68%, slightly over the expected percentage for a full first cousin once removed relationship and double that of her brother. This was a bit of a relief, but still does leave some question in my mind about whether, in reality, we share both of our great grandparents or only one. I am attempting to get a male Moore cousin descended from my grandfather's brother to test to make sure that he shares my father's and uncle's unusual Y-DNA haplogroup of I2b1.

To more clearly demonstrate the randomness of autosomal DNA inheritance, I compared this paternal cousin to each of the second cousins and their offspring. The second cousins once removed are expected to share 1.563% of their DNA on average, but, in reality, they share 1.26%, .92% and .33%. When you see this, you can begin to understand why at the next step, the third cousin level,  approximately ten percent of the time there is not enough shared DNA to detect a familial relationship.

Look at the chart comparing Second Cousin #1 and her offspring to PCousin. Do you see something that doesn't make sense?

The surprising thing is that the offspring (2nd cousin1xR #1) of the sister (2nd cousin #1) who shared the least with our paternal cousin, shared the most DNA of the three second cousins once removed. In fact, she shares more than her mother does (95 cMs vs 86 cMs) and appears to share an additional segment on Chromosome 22 with our paternal cousin that her mother does not share. Since there is definitely not any shared ancestry on 2nd cousin 1xR #1's father's side with this cousin and I cannot compare my deceased father to see if the match shows up in him, I looked at the both of her aunts to try to determine if they also shared this segment with PCousin and for some reason it just isn't showing up. 

Comparing Second Cousin1xR #1 with her aunt (Sister #2)

Sure enough, there it is on Sister #2's chart. This shows that for some unknown reason, the match did not show up in Sister #1's chart, but it must be there. Perhaps a no-call is breaking the segment into two, thus making it appear to be too small to be of significance.

[**Update - Dr. Ann Turner emailed me about this scenario writing, " are treated as if they match, so that's not an explanation. It could be a miscall or a 'fuzzy boundary,' where the child has inherited enough alleles from the other parent to make it appear that the long consecutive run of SNPs is continuing."  She asked if I have access to the raw data for this cousin (which I do not) and further suggested, "If so, you could run David Pike's utility with more liberal parameters." This explanation had occurred to me, but the fact that the match was over 10cMs and appeared in both individuals of different generations led me to believe that was not a possibility. I will update if I learn anything further.]

Regardless, it is exciting to know that these shared segments come from one or both of our shared great grandparents. This realization certainly makes me feel closer to my ancestors that I have invested so much time trying to get to know through my family history research.

Willard and Blanche Moore - It's their DNA! 


  1. CeCe, I love these posts comparing the DNA among your sisters and cousins. It really helps make sense of the concepts.

  2. This is interesting! Do you have a chart or table showing the expected percentage of DNA sharing for the various levels of cousins?

    My sister has just tested and so has her husband (who we found out years ago that he is a distant cousin - fourth cousin I think).

  3. I sure do, Joan.
    There is a ~50% chance of detecting their shared DNA if they are fourth cousins.
    Thanks for the comments, ladies!

  4. On further review, I suspect the segment on chromosome 22 missing in sister #1 is just below the threshold for declaring a match. The fact that the segments in the cousin and sister #2 are slightly offset *might* be a sign of fuzzy boundaries, although it would require access to raw data to analyze the details.

  5. well, i could only contact my half-sister to submit a sample, which she did but does not in other ways participate, so i have a little info to compare, so i am both envious and interested on a scientific anthropological perspective of your discussion

  6. Thanks, Ann.
    @MEJ- Good luck with your research and thanks for commenting.

  7. I have a question, say my great-grandmother had two children boy and girl but from different fathers which would make them haf brother and sister. Now the boy grows up and has a daughter and also ther boys sister grows up and has a doughter, which would make the children half first cousins. Now the two first falf cousins also have chidren, then would they be half second cousins? and also how close in terms of blood relation would they be, would they be considered related at all, although they share a great grand mother?

  8. I have a question,now suppose my great-grandmother has two children but from different fathers, then theywould be half brother and sister. Now the children grow up and also have children, the brother has a doughter and the sister has a daughter, then the two children become haf first cousins. Now the half first cousins also have children, one has a boy and the other has a girl, would they be half second cousins? and how close will they be in terms of blood relation, would they be really your relatives and could it be possible to have a relationship with them? I hope i'm making sense and not confusing anyone, plese help?

  9. I have a question,if my great grandmother had two children but from different fathers a boy and a girl,then the two children grow up and also have their own children who are both girls,which would make them half first cousins. Now the two girls grow up and also have children,one having a boy and the other having a girl,would they then be half second cousins to each other and would they be closely related by blood and would they be from the same generation?