Over on the Rootsweb Autosomal DNA List a debate is raging in regard to the chances of randomly matching an actual third cousin in the autosomal DNA databases at 23andMe or Family Tree DNA. Some feel that this is extremely unlikely and a number of our top atDNA genetic genealogists are weighing in on the subject. I have been following this intriguing discussion for a couple of days now and hoping that the astronomical odds quoted are overly pessimistic. With this is mind, I was especially excited to randomly discover and confirm an actual third cousin match today from 23andMe's Relative Finder.
Honestly, I am so busy helping other people with their autosomal DNA research that I rarely have time for my own. However, today I was quickly scanning my paternal uncle's new Relative Finder matches and I saw a predicted 2nd-3rd cousin.
Happily, this cousin accepted my invitation to compare our DNA and told me that he had the Moore surname in his family tree. When I looked up his unusual name on Facebook, I found that we had a mutual friend - the wife of my third cousin once removed through my Moore great great grandparents. She quickly filled me in on my match's genealogy and I discovered that we are, indeed, third cousins sharing our mutual great great grandparents Calvin and Mary/Martha (Armstrong) Moore. Of course, we don't share that much DNA at that level, only .43% of our DNA matches each other (3rd cousins would be expected to share about .781%). My sisters are also well below the expected amount at .52% and .30% and my second cousin from this line shares only .31% with him. My father's two siblings who are his second cousins once removed share 1.13% and .93% (expected ~1.563%). These numbers are a little lower than expected, but this isn't altogether surprising since autosomal DNA inheritance is so random and, especially after the second cousin level, I have found it to be pretty inconsistent. Regardless, the prediction by 23andMe was right on.
Since I don't share that much DNA with this third cousin, it was fairly easy to overlook him in my match list:
Given this opportunity, let's look a little closer at my family comparisons with this cousin. The chart below graphically illustrates the DNA that my sisters and I share with our 3rd cousin. Each gray bar represents a pair of chromosomes. The colored bands represent where each of my sisters and I have a matching segment of DNA with our cousin. As you can see, my sisters and I each inherited unique patterns of our great great grandparents' DNA. Much of the shared DNA is on Chromosome 8, especially at the end where all three of us inherited the same small DNA segment from our Moore 2nd great grandparents.
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| Third Cousin Comparisons - Click to enlarge |
Now, let's look at the inheritance pattern of two second cousins once removed. This graph shows the areas where my dad's siblings match this Moore cousin. They share twice as many segments as two of us third cousins and three times as many as my sister who has the least DNA in common with them. Notice again, how little overlap there is between the two and that there is very little matching on Chromosome 8. This means that my dad's inheritance pattern was equally as unique since we sisters all show considerable matching on that specific chromosome with our Moore cousin.
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| 2nd Cousin Once Removed Comparisons - Click to enlarge |
Next, we can see that our second cousin (also a third cousin to this newly discovered Moore cousin) inherited completely different DNA from our shared 2nd great grandparents than my sister and I. Also, she only has two segments in common with him (light blue), but one is larger than any of those my sisters and I share with him. Her overall sharing is lower at only 23 cM versus 32 cM for me and 39 cM for my sister. She is more in line with my other sister (not shown here) who only shares two segments and 22 cM with this cousin.
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| More Third Cousin Comparisons (Click to Enlarge) |
As I have explained before, I tested my father's siblings since he is deceased, hoping to be able to use them as a proxy for his DNA. Since siblings only share an average of 50% of the DNA, you can see this plan does not always work out. My uncle (dark blue) shares almost completely different DNA with this cousin than my sister and I do. The only commonality with this "new" Moore cousin lies on Chromosome 11 between my uncle and sister (illustrated by light blue and dark blue bars).
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| One 2nd Cousin Once Removed versus Two 3rd Cousin Comparisons |
The same thing holds true for my paternal aunt (dark blue bars below). You can see all three of us match this cousin only on a small segment on Chromosome 8 (stacked colored bars). Since the dark blue on the chart below does not overlap any of the other DNA matches that my sister and I have with our Moore cousin, it is evident that my father's pattern of inheritance from these specific ancestors was quite diverse from that of both of his siblings.
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| Another 2nd Cousin Once Removed versus Two 3rd Cousin Comparisons |
Since Calvin and Mary Moore are the sole common ancestors shared with this newly discovered cousin, then we can deduce that all of the shared DNA in those charts above originates with them. For me this means that this specific DNA is inherited from their son Willard Calvin Moore, my great grandfather. I can now identify these three little spots - one on Chromosome 1 and two on Chromosome 8 - as a gift from Willard to me.
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| Willard Calvin Moore (1877 - 1934) |
[**Update - I have been receiving lots of inquiries about Mary "Martha" Armstrong Moore, so I will post her basics here. She was born on 2 May 1836 in Newport, Ontario, Canada to recent English immigrants Thomas and Dorothy (Hudspith) Armstrong from Northumberland, England. Mary had 13 siblings. She married Calvin Benjamin Moore in Ontario in 1854 and shortly thereafter moved to Michigan where she had, at least, eight children before dying in 1878 of consumption when my great grandfather Willard was only one year old. I would love to find an Armstrong cousin, so if this sounds familiar, please drop me a line!]
