Comparing Cousins

Part of my research I have been slacking on posting about is my ongoing adventure with DNA testing. In addition to my mother’s autosomal testing, her third cousin has completed autosomal and Y DNA testing now (including two separate single SNP tests, S17250 and Y4460), as well as a second cousin along the Akerfelds line and now myself! I recently took an autosomal test and am excitedly awaiting the results.

The autosomal tests between the 3 cousins so far definitely show them as being related, within the correct estimated ranges of shared DNA for 2nd and 3rd cousins. Interestingly, the 2nd cousin is sort of a “double cousin” to my mother, as her grandparents (Janis and Anna Akerfelds) are brother and sister to his grandparents (Arturs and Katte Ziverts), meaning they share 4/8 great grandparents, while 2nd cousins usually share 2/8. This means they should have even more DNA in common, and they are indeed in the higher range of values for a second cousin relationship. What else is interesting is my mother and her 2nd cousin share more total DNA, across many more smaller segments, while my mother and her 3rd cousin (he shares 2/16 great great grandparents) share a much longer unbroken segment (57cM). My interpretation of that is that longer segments shared between two people indicate a strong inheritance from less people while a greater number of smaller segments shows two people share many of the same more recent ancestors – this could either mean two people are closer cousins and therefore share more ancestral lines OR they may be more distant cousins along several different family lines without knowing it.

Having these other tests to compare known relationships to is handy when looking at unknown cousin matches. Size and amount of DNA segments can be compared to interpret genetic distance, or to compare other unknown matches and where they share DNA segments – chances are good a shared segment amongst two known cousins and a third unknown could all come from the same ancestor, same family line. The 3rd cousin only shares one known family line with my mother and the 2nd cousin – Akerfelds, while my mother and her 2nd cousin share Akerfelds, Ziverts, Dzerve and Bitners – her entire paternal side and his entire maternal side (which is his only Latvian side, his father is a different ethnicity, making it even easier because this tidbit means that ANY Latvian cousin matches he has is from a line he shares with my mother).

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Chromosome browser for my mother and the two cousins mentioned above. Orange is the 2nd cousin while blue is the 3rd cousin. Note the long matching segment on chromosome 7 shared with the 3rd cousin, and the multiple smaller segments shared with the 2nd.

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