atDNA: I Don't Match My Cousin's Cousin

I've had several questions asking what may cause one person's autosomal DNA (atDNA) test to match one cousin, but not another who shares the same ancestor. This is a fairly common occurrence and is easily explained.

The autosomes, chromosomes 1 through 22, are a randomly recombined mix of the DNA our parents inherited from our grandparents. The random recombination causes the segments passed to children to sometimes vary in length. One child may get a long segment. Another child may get a smaller segment or several smaller segments. And with each new generation the segments can be further ... well, further segmented. Each testing company has a minimum threshold for the size of a segment of DNA two people must share before they are listed as a match.¹

The chart below shows one chromosome segment of three cousins, DD, NN, and MM. The numbers represent the location of SNPs on chromosome 6.

DD and NN share two segments on chromosome 6: one segment starts at location 18,364,604 and ends at 32,432,923, there is a break where DD's DNA does not match NN, then a second segment starts at location 32,707,977 and ends at location 36,812, 814. These two segments are 12.84 and 5.85 centimorgans long, respectively, totaling 18.69 centimorgans.² The 12.84 centimorgan segment exceeds the threshold for minimum segment size for DD and NN to be considered a match.

MM shares one segment with NN starting at location 29,684,571 and ending at 37,854,826 totaling 9.13 centimorgans. This MM:NN segment overlaps the end of the 12.84 centimorgan segment shared between DD and NN, includes the segment where DD and NN do not match between the two matching segments, the 5.85 centimorgan segment shared between DD and NN, and another segment not shared by DD and NN that follows the 5.85 centimorgan shared segment. The 9.13 centimorgan segment exceeds the threshold for minimum segment size for MM and NN to be considered a match.

MM and DD only share two segments, each smaller than 6 centimorgans in length. Neither of these segments is long enough to meet the threshold for minimum segment size for DD and MM to be considered a match at the testing company.

To be sure that MM and DD share DNA in common with NN, and therefore likely got it from the same ancestor, requires access to the raw data and the detailed segment data. Today, we only get this information from
23andMe and Family Tree DNA atDNA tests. Some third-party analysis tools also allow analysis beyond what can be done at a testing company, but we need to be sure we understand the privacy policies in place at the third-party site before we use the tools there.

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1. "Family Finder versus Relative Finder," ISOGG Wiki (http://www.isogg.org/wiki/Family_Finder_versus_Relative_Finder#Thresholds_for_relationship_matches : accessed 22 September 2013), see section titled "Thresholds for relationship matches."
2. "Centimorgan," ISOGG Wiki (http://www.isogg.org/wiki/Centimorgan : accessed 22 September 2013)

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To cite this blog post:
Debbie Parker Wayne, "atDNA: I Don't Match My Cousin's Cousin," Deb's Delvings Blog, posted 22 September 2013 (http://debsdelvings.blogspot.com/ : accessed [date]).

© 2013, Debbie Parker Wayne, CG, CGL, All Rights Reserved