Monday, October 31, 2011

Investigating Another Genealogical Theory Using DNA - Ratekin

Recently a researcher of the surname Ratekin contacted me, explaining that his brother had seen one of my postings about my Ratekin ancestors. I descend from Patrick David Ratekin (c.1740-1804) and he descends from his presumed brother John Ratekin (c.1732-c.1806). Although there is circumstantial evidence supporting the belief that Patrick and John were brothers, to my knowledge no proof has been found. Both of these Ratekin men lived first in Berks County, Pennsylvania, then moved to Loudon County, Virginia and finally settled in Campbell County, Virginia. If they were indeed brothers, then this Ratekin Cousin and I would be 7th cousins.

I was excited to find out that he had also tested his DNA at 23andMe. So we "shared genomes" match! Unfortunately, we weren't able to confirm the relationship between us using DNA, however this does not disprove the theory that these two were brothers. Why is this? It is because the odds of 7th cousins having enough shared DNA to be detected by 23andMe's ancestry tools are extremely low - well under five percent.

Autosomal DNA is a very useful tool for proving relationships, however for relatives beyond second cousins, the lack of shared DNA cannot disprove a familial relationship. It is estimated that even an authentic 3rd cousin relationship will not be detected by 23andMe's ancestry tools about ten percent of the time. This is because of the random nature of autosomal DNA inheritance. With each successive generation, the DNA is spliced, mixed up and recombined. As this happens it is possible, although rare, for the DNA of a great great grandparent to completely disappear from our genome (at least as far as the current tools are concerned). By the time you reach a common ancestor as distant as a sixth great grandfather, the chances are good that all detectable traces of that ancestor have disappeared from your DNA. This does not mean that it is impossible to discover an authentic 7th cousin, or even one more distant, at 23andMe or on FTDNA's Family Finder. (In fact, I have done so.) Since all of our DNA necessarily must come from our ancestors, it is not surprising that we can still find traces of some of the more distant ones. Unfortunately, a majority of the time this will not be possible.

So, this time the investigation did not provide us with any useful data, but I am looking forward to the next opportunity for discovery.

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