I'd already tested my parents but one of my sons has now also taken the Family Finder test which gives me the opportunity to explore the inheritance patterns of these shared DNA segments in more detail.
In the screenshot below I've compared my dad with Mr K. They are third cousins once removed. They share three large segments in common: 20.12 centiMorgans on chromosome 1; 23.33 centiMorgans on chromosome 3 and 17.12 centiMorgans on chromosome 11.
The screenshot below is taken from the perspective of my son, and I've set the threshold to 1 cM. The chromosome browser shows the segments my son shares in common with me, his maternal grandfather and his cousin Mr K. The segments shared with Mr K are shown in orange. The segments he shares with his grandfather are shown in green. The blue segments are shared in common with me. A child receives 50% of his DNA from his mother so my son matches me across the entire length of each chromosome. (Note that for each of the 22 autosomes we receive one entire chromosome from our mother and one entire chromosome from our father but the chromosome browser can only show matches on one of these two chromosomes.) There are 13 small segments that my son appears to share with Mr K. However, ten of these segments are seemingly shared by me, my son and Mr K but are not shared with my father. Clearly this is a biological impossibility because if a segment is identical by descent (IBD) then by definition it must have been passed on from a parent to a child and it couldn't possibly skip a generation. There are tiny segments on chromosome 6, chromosome 10 and chromosome 16 that are shared by all of us and these segments are therefore more likely to be IBD.
triangulation (identifying three or more segments which match on the same chromosome) confirms that the segments are "real" or in other words that they are identical by descent (IBD). In this case the 13 small segments all triangulate with three people - me, my son and Mr K. However, when my dad is added to the mix we can see that the triangulation process breaks down. If the small segments were IBD then my dad should match on all of these small segments.
In the future when whole genome sequencing becomes the norm it should be possible to use small segments for genealogical matching purposes but with the limitations of the current technology extreme caution should be used when drawing conclusions about matches on small segments.
The ISOGG Wiki article on identical by descent has further information on this subject:
There have been a number of blog posts that have dealt with the subject of small segments and they are all linked on the ISOGG Wiki page. I particularly recommend reading the following:
- Genealogy and autosomal DNA matches: common errors in “proving” an ancestor, and the allure of easy gateway ancestors by "Our Puzzling Past"
- What can the X chromosome tell us about the importance of small Segments? by Kathy Johnston (a guest post on Kitty Cooper's blog)
- Small matching segments - examining hypotheses by Blaine Bettinger
- What a difference a phase makes by Ann Turner (a guest post on Blaine Bettinger's blog)
I received a free DNA test from Family Tree DNA in compensation for speaking at Who Do You Think You Are? Live in 2014. I chose to have a Family Finder which I used to test my son.