It can be tricky determining if small segments of shared DNA (~5.0cMs) imply true relatedness or are simply "noise". It is possible that matches of this length are, in fact, Identical By State (IBS) and not Identical By Descent (IBD). In simpler terms, this means that the match could have happened by coincidence and does not mean that there is a common ancestor. Some experts have even predicted that up to one-third of these reported small matches are false positives.
However, in my opinion, sharing
multiple small segments (> 5 cM) of DNA in common with another person does imply relatedness because the odds decrease dramatically that the coincidence of a greater than 5 cM match could occur more than once. With this in mind, I often investigate my small 23andMe matches further to see if there are more segments in common that fall just below 23andMe's reporting threshold for Family Inheritance Advanced. If my "match" has submitted his/her data to independent projects such as Jim McMillan's
DNA Cousins Project (must be logged into 23andMe to access) or
HIR Search, then I can easily determine this.
As an example, for one person of Finnish ancestry (FM), 23andMe's Family Inheritance Advanced reports a match to my mother on two small segments (5.0 cMs x 2; Chr. 4, 5), my mother's sister (5.0 cMs; Chr 7) and my sister (5.5 cMs; Chr 9).
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| Chart from 23andMe's Family Inheritance Advanced |
These matches all fall on different chromosomes, which on the surface doesn't make sense since my sister had to have inherited her segment from my mother because all of our Finnish matches are, undoubtedly, through her. Since the match is hovering on the border of 23andMe's cut-off, I am betting that my mother does in fact share the match on Chromosome 9 with my sister.
HIR reports 10 matching segments between my mother and FM ( > 5 cMs and 500 SNPs). Most of these were not identified by 23andMe due to the SNP count falling below 700. As I suspected, one of these segments is on the same spot of Chromosome 9 that was reported at 23andMe for my sister. HIR calls it at 5.8 cMs for my mother, which is close to the 5.5 cMs that 23andMe reported for my sister, but my mother only shares 638 SNPs on HIR, while on 23andMe, 872 matching SNPs were reported for my sister. This may be an instance where my sister coincidentally gained a strand of matching SNPs from my father's DNA that made the match appear a bit longer (IBS).
23andMe and HIR agree on my mother's matches with FM on Chr 4 and Chr 5. HIR calls them a bit longer, but not significantly so. The match that my aunt shares with FM is not present in my mother's DNA. Interestingly, HIR reports a 7.7 cM match on Chromosome 22 for my mother and FM, but with only 529 SNPs. Notably, 23andMe did not report any matches between FM and myself, while HIR shows 3 matches ( > 5 cMs and 500 SNPs). As would be expected, those matches are all in common with my mother though not completely consistent in reported size. Somewhat surprisingly, two of the matches reported by HIR between FM and myself, by their calculations, exceeded 23andMe's threshold: 1) 6.1 cMs and 1261 SNPs and 2) 5.2 cMs and 1781 SNPs. It is possible that HIR has a higher tolerance for mismatched SNPs especially since, in both cases, HIR reported more matching cMs and SNPs with FM for me than for my mother.
The next step is to share family trees and attempt to find our common ancestor(s). It may be that we share multiple common ancestors that have each contributed one or two of these segments, in which case the connection may be too far back in time to identify. I'm looking forward to finding out.
