Tuesday, October 31, 2017

DNA of Preston Sheehy Descendants at 23andMe

Another Preston/Sheehy cousin tested his DNA at 23andMe.com. K.J. is the father of D.J., who tested four years ago. D.J. was the first relative I identified from my DNA pursuits.

Below is the family tree of the descendants of John Preston (1857-1928) and Bridget Sheehy (1857-1916) who tested at 23andMe.

Below is the chromosome browser view of the shared DNA between K.J. and his three second cousins. Note the wide variation in the amount shared from 195 centimorgans (cM) to 364.

Also important is that the amount of shared DNA drops when we shift from father to son. The amount was not halved; it was quartered. (This is why it is best to test members of the oldest living generation whenever possible.)

23andMe has an "In Common With" feature. This list shows DNA testers who match you and a target person. In the scenario below, I looked for relatives in common with my mother and K.J. Sharing the same DNA indicates that the DNA tester is likely from their shared Preston/Sheehy lines. One DNA tester, R.S., could be a viable lead.

R.S. shares a segment of DNA with the oldest generation of Preston/Sheehy descendants. The chromosome browser reveals that my mother shares the longest segment and this segment broke in the middle. My uncle received one piece while K.J. and my mother's first cousin received the other piece.

R.S. has no family tree offered through 23andMe. This is a common problem with matches at 23andMe- the lack of genealogical information and interest.

The other observation to garnish from this information is that the amount of shared DNA skews greatly beyond the parent - child relationship. In groups on FaceBook, I often see people trying to determine generations or half relationships based on the amount of shared DNA of people well beyond a sibling relationship. You simply cannot do this based on shared DNA alone.

Please see the latest Shared Centimorgan Project by Blaine Bettinger for the ranges of DNA shared by relatives up to a fourth cousin. The numbers found in my cousin comparisons fall within these expected amounts.

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