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.
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, "...no-calls 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 http://www.math.mun.ca/~
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!|