They Have the Same 5xGreat Grandfather in Their Tree - We're a Match!!!
Perhaps by now you have been exploring Ancestry's "shaky leaves", the little icon that serves notice of a Shared Ancestor Hint in the tree of one of your Matches. And how exciting - they too have a John Smith as a 5x great grandfather (5G), and Ancestry says that you are 6th cousins (6C)! Genealogy and genetics are in agreement - huzzah!
Well, maybe! The amount of the match is pretty small; clicking on the little i in a dark circle on the match page shows that you match at an amount 8 centiMorgans (cM). Since this is above Ancestry's 6 cM threshold for significant match possibilities, they have showed it to you. But….
Each generation doubles in number, and in theory we'd get equal amounts of DNA from each member of that generation:
• 2 parents (50% each)
• 4 grandparents (25%)
• 8 great grandparents (12.5%)
• 16 great-great grandparents 2Gs (6.25%)
• 32 3Gs (3.125%)
....and so on. If we go back in time 10 generations, our theoretical tree includes 1,024 8G grandparents contributing approximately 0.1% of your DNA each!
It is safe to say that not many people can confidently know each of those 1,024 branches of their family tree back that far. We may well be limited by the available records. The farther into the past we research, the higher the likelihood that we encounter "brick walls." Whatever records were kept at the time may have since been lost in fire or flood or some other calamity. Other vital events (birth, marriage and death) may simply not have been recorded by a church or civic authority. Adding to our genealogical challenge, the same ancestors may show up in multiple branches of your family due to multiple marriages between several families that was commonplace in many small communities up until at least the mid-19th century.
In reality, we don't get equal amounts from our different ancestors. Current research shows that less than 10% of 6Cs have any autosomal DNA in common at all above a statistically reliable threshold. There is general consensus that matching segments smaller than 7 cM have too great a chance of being falsely positive, and should be ignored in the absence of other supporting evidence. Many genealogists use a higher threshold of 10 or 12 cM to be reliably considered a match.
In practice, the likelihood of positively identifying a common ancestor with matches at less than 20-30 cM can be difficult. Your common ancestor could be as far back as 8-12 generations and that is a long way for both you and your match to have positively traced your tree. That common ancestor was probably born sometime between the mid-1600s to early 1700s. Additionally, matches at that amount could result from a wide range of relationships spanning several generations. So if our theoretical 6C from John Smith does share a statistically significant match with us, it is a bit of luck. To positively determine that John Smith is indeed the common ancestor is more luck and a bunch of work.
My dad has over 200 matches with Shared Ancestor Hints. Most of them are total matches of less than 20 cM. Nearly 60% of those matches are between 6 and 10 cM. Other than the 4th cousins and closer matches with known shared ancestry, those Shared Ancestor Hints come exclusively with people who appear to share colonial New England ancestry from the mid to late 1600s. DNA from those few thousand early settlers has been inherited by tens of millions of descendants across the continent. It stands to reason that sharing DNA with as few as 1 in 100 of their descendants would still result in lots of matches, even across four centuries and a lot of generations.
But, just because someone has the same distant ancestor in their tree AND you share a meaningful amount of DNA does not confirm that relationship. While it's possible that you got that segment from one of your 128 5Gs or 256 6Gs who happens to also be one of your match's ancestors, it is entirely possible that you inherited that strand from a different common ancestor unknown to you both. It is evidence, but far from validation. That requires a more sophisticated analysis known as triangulation. And some luck :)
Well, maybe! The amount of the match is pretty small; clicking on the little i in a dark circle on the match page shows that you match at an amount 8 centiMorgans (cM). Since this is above Ancestry's 6 cM threshold for significant match possibilities, they have showed it to you. But….
Each generation doubles in number, and in theory we'd get equal amounts of DNA from each member of that generation:
• 2 parents (50% each)
• 4 grandparents (25%)
• 8 great grandparents (12.5%)
• 16 great-great grandparents 2Gs (6.25%)
• 32 3Gs (3.125%)
....and so on. If we go back in time 10 generations, our theoretical tree includes 1,024 8G grandparents contributing approximately 0.1% of your DNA each!
It is safe to say that not many people can confidently know each of those 1,024 branches of their family tree back that far. We may well be limited by the available records. The farther into the past we research, the higher the likelihood that we encounter "brick walls." Whatever records were kept at the time may have since been lost in fire or flood or some other calamity. Other vital events (birth, marriage and death) may simply not have been recorded by a church or civic authority. Adding to our genealogical challenge, the same ancestors may show up in multiple branches of your family due to multiple marriages between several families that was commonplace in many small communities up until at least the mid-19th century.
In reality, we don't get equal amounts from our different ancestors. Current research shows that less than 10% of 6Cs have any autosomal DNA in common at all above a statistically reliable threshold. There is general consensus that matching segments smaller than 7 cM have too great a chance of being falsely positive, and should be ignored in the absence of other supporting evidence. Many genealogists use a higher threshold of 10 or 12 cM to be reliably considered a match.
In practice, the likelihood of positively identifying a common ancestor with matches at less than 20-30 cM can be difficult. Your common ancestor could be as far back as 8-12 generations and that is a long way for both you and your match to have positively traced your tree. That common ancestor was probably born sometime between the mid-1600s to early 1700s. Additionally, matches at that amount could result from a wide range of relationships spanning several generations. So if our theoretical 6C from John Smith does share a statistically significant match with us, it is a bit of luck. To positively determine that John Smith is indeed the common ancestor is more luck and a bunch of work.
My dad has over 200 matches with Shared Ancestor Hints. Most of them are total matches of less than 20 cM. Nearly 60% of those matches are between 6 and 10 cM. Other than the 4th cousins and closer matches with known shared ancestry, those Shared Ancestor Hints come exclusively with people who appear to share colonial New England ancestry from the mid to late 1600s. DNA from those few thousand early settlers has been inherited by tens of millions of descendants across the continent. It stands to reason that sharing DNA with as few as 1 in 100 of their descendants would still result in lots of matches, even across four centuries and a lot of generations.
But, just because someone has the same distant ancestor in their tree AND you share a meaningful amount of DNA does not confirm that relationship. While it's possible that you got that segment from one of your 128 5Gs or 256 6Gs who happens to also be one of your match's ancestors, it is entirely possible that you inherited that strand from a different common ancestor unknown to you both. It is evidence, but far from validation. That requires a more sophisticated analysis known as triangulation. And some luck :)
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