Thursday, November 28, 2013

Third Cousin Comparison - FamilyTreeDNA Family Finder Autosomal DNA test

Back in July I made a request on this blog, asking if any of my known cousins would be interested in taking the Family Tree DNA (FTDNA) Family Finder test. I was very happy that two of my Dad's third cousins (through the Fullerton family) decided to take the test. I want to thank both of them very much and I hope I can help them in some way to understand the results.

Chromosome Browser

What follows are screen shots of the Chromosome Browser at FTDNA indicating the segments of DNA where my Dad and I match with his third cousins (names have been removed for privacy and replaced with 'Cousin A' and 'Cousin B')

FTDNA Chromosome Browser: A comparison of DNA segments over 5 cM shared between my father and two third cousins. Their common ancestors are William Fullerton/Fullard (c1815-1869) and Mary Dunn (c1813-1858) who were married in Clane, County Kildare, Ireland in 1839 and immigrated to Australia in 1840. Please see my previous post Finding The Fullertons for more information about this couple and their family. 
FTDNA Chromosome Browser: A comparison of DNA segments over 5 cM shared between myself and two third cousins once removed. Our common ancestors are William Fullerton/Fullard (c1815-1869) and Mary Dunn (c1813-1858).

Shared Segments

The Autosomal DNA statistics page on the ISOGG (International Society of Genetic Genealogy) Wiki states that third cousins share a mathematical average of 0.718% of DNA with an average of 53.13 shared centiMorgans (cMs) .

My Dad and 'Cousin A' (Third Cousin) Shared Segments
  • A total of 19 segments shared 
  • 170.21 cM (centiMorgans) total shared over those 19 segments 
  • 8 segments greater than 5 cM and 141.5 cM total shared over those 8 segments greater than 5 cM - the smaller the segment the more likely it is to be Identical by State (IBS) rather than Identical by Decent (IBD)
  • 6 segments greater than 10 cM. 
  • Largest shared segment of 30.1 cM.
My Dad and 'Cousin B' (Third Cousin) Shared Segments
  • A total of 9 segments shared 
  • 73.91 cM total shared over those 9 segments 
  • 3 segments greater than 5 cM. A total of 58.26 cM total of shared segments greater than 5 cM. 
  • 3 segments greater than 10 cM. A total of 58.26 cM total of shared segments greater than 10 cM. 
  • Largest shared segment of 22.89 cM.

Triangulation (or 'Matches in common')

Interestingly, while my Dad and I matched both cousins, 'Cousin A' and 'Cousin B', who are also third cousins to each other, did not meet the criteria to be considered matches at FTDNA. They obviously both inherited different DNA from their common ancestors, while my Dad and I happened to inherit a combination of segments of DNA from those ancestors DNA as both cousins. FTDNA's FAQs includes the question What is the probability that my relative and I share enough DNA for Family Finder to detect? The answer states that about 10% of third cousins will not share enough DNA to be considered a match according to FTDNA's criteria. However, these cousins may find that they do actually share smaller segments of DNA, for example if they utilise some of the comparison tools available at Gedmatch.com.

Matches shared in common with my Dad and 'Cousin A'
  • 4 matches in common between my Dad and 'Cousin A' (5 if you include me) 
  • Of these matches in common, 2 match on a segment that overlaps with a segment where 'Cousin A' matches my Dad.
Matches shared in common with my Dad and 'Cousin B'
  • 5 matches in common between my Dad and 'Cousin B' (6 if you include me) 
  • Of these matches in common, 2 match on segments that overlap with a segment where 'Cousin B' matches my Dad.
Now I need to contact these matches and explore further to see how they might be related through my Fullerton or Dunn ancestors. I am also hoping to ask some of my Dad's closer cousins if they would be willing to take the Family Finder test. This would give more data to work with.
Very interesting so far.
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