Tracking DNA segments through time and space

One of the exciting aspects of autosomal DNA testing is that it gives us the opportunity of assigning segments of DNA to specific ancestors and then tracking the inheritance of those segments over time. To date the only match where I've been able to make the genealogical connection other than with immediate members of my family is with a fourth cousin, Mr K, who lives in Canada. I wrote previously about this match in my article on My first autosomal DNA success story. That match was very easy to identify because all my ancestors are in the UK and all Mr K's ancestors are in Canada and there was only one possible line where we could connect. It also helped that one of the shared surnames - Cruwys - is very rare. We can therefore state with confidence that the segment we share has been inherited from our ancestors William Cruwys (1793-1846) and Margaret Eastmond (1792-1874) who married in Rose Ash, Devon, on 18th July 1814.

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.

Next I used the chromosome browser to compare myself with Mr K. Mr K and I are fourth cousins. You can see that two of the segments that my dad inherited have not been passed on to me and I only share a single segment on chromosome 11 with Mr K. This segment is 16.62 cMs and has been passed on virtually intact from my father to me.

The next screenshot shows a comparison with my son and Mr K. They are fourth cousins once removed. You can see that my son has inherited exactly the same segment as me. This segment measures 16.85 cMs and appears to be slightly larger in my son than it is in me, which is perhaps something to do with the rounding that FTDNA uses.

For all the above screenshots I've set the threshold at 5 cMs. Family Tree DNA are the only company who provide segment data under the 5 cM threshold. There has been much debate in the genetic genealogy community on the subject of small segments under 5 cMs, but there is a consensus that the vast majority of the small segments generated by the current chip tests are false positives and are nothing more than random noise. However, now that I have tested three generations of my family and we also have a match with a known cousin, I thought I would take the opportunity to do a comparison of the small segments to satisfy my own curiosity.

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 chromosomes come in pairs - we receive one set of chromosomes from our mother and one set of chromosomes from our father. However, the chromosome browser shows matches on a single chromosome and is unable to identify whether the match is on a maternal or a paternal chromosome.) 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.

There have been suggestions that the process of 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.

Further reading
The ISOGG Wiki article on identical by descent has further information on this subject:

http://www.isogg.org/wiki/Identical_by_descent

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:

- The folly of using small segments as proof in genealogical research by CeCe Moore

- Genealogy and autosomal DNA matches: common errors in “proving” an ancestor, and the allure of easy gateway ancestors by "Our Puzzling Past"

- Chromosome Pile-Ups in Genetic Genealogy: Examples from 23andMe and FTDNA 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 - friend or foe? by Blaine Bettinger

- 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)

Disclosure
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.

© 2015 Debbie Kennett