Tuesday, August 6, 2013

Gedmatch Demonstration Free DNA Comparison

Gedmatch is a great way to compare your DNA results with likely cousins. However, there are no hard and fast rules when comes to DNA, which can be tricky when it comes to establishing relationships. We are all descended from two individuals who lived in Africa thousands of years ago as the scientist Spencer Wells has proven. Also, more recently, the population of Europe was much smaller up until modern times. As a for instance,  if those with European ancestry trace their Family Trees back far enough they will notice they are related to the same people more than once. This is true of other parts of the world too. So sometimes when we see we share small bits of DNA segments with someone else we could be related hundreds of years ago, or, even thousands of years ago.
Another problem when trying to make connections with cousins is not all the DNA our parents inherited is passed down to us. Our siblings may have inherited different segments than we have. Since we don't inherit all of our parents DNA we can lose all traces of our early ancestors DNA eventually.

Here is a demonstration of how Gedmatch can confirm our paper trail ancestry. Here we are doing one on one kit comparisons looking for matching segments of DNA.
Nan and I have a paper trail proving our relationship as 5th cousins. We compared our DNA at Gedmatch. Here are the results:

Nan and Annette compared
Notice that Nan shares over 2,000 SNP's on each shared segment with me. This is a tip off that we are related to someone in the genealogical time frame.
I have another cousin, Sharon, who is from the same Forgey family whose results matched Nan and I. We don't have a paper trail for her relationship to us. We know through Y- DNA testing that we are cousins. She might be a 7th cousin but we're not sure? This is how she matched both Nan and I:

Annette and Nan compared to Sharon
You can see how we both share fewer SNP's on our shared segments as we lose this early generations ' DNA traces. 
Another cousin of hers, Melanie, also tested and neither Nan or I had a single matching segment with her. Sharon and Melanie have a paper trail confirming they are cousins. 
Here we have the 6th cousins Sharon and Melanie comparing:

Sharon and Melanie Compared

They share a very large DNA segment, and a whopping 4,000 SNP's which means there is no doubt they are related.
If you are comparing your DNA to someone you have a suspicion you are related to, and maybe a paper trail, or share the same surnames with, or your families lived in the same area, you may want to turn down the 700 SNP result to 400 or 500. If you don't have anything pointing to a relationship you might want to leave the SNP's at 700.
Here is an explanation of why Gedmatch sets it's results at the level they do:

"To qualify as a 'match' in the genealogical time frame, results must have a largest Autosomal segment that has at least 700 SNPs and be at least 7 cM.
It must have BOTH. Results with the largest segment less than 7 cM are highlighted in pink. To check the number of SNPs, click on the 'A' on the same line to view the one-to-one comparison detail."

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