…And What Your Results Don't Mean
In my last post we covered what we can learn from our DNA matches. This post is just as important, for there are many limitations.
We don't inherit the same amounts of DNA from each of our 16 great-great grandparents (2Gs). As we go farther back through successive generations, we find that we don't have any identifiable DNA from many different lines in our family, while DNA from a few branches of our family may still show up 10 to 12 generations distant.
Autosomal DNA has been found to be reliable over 5 to 7 generations. A 3rd cousin (3C) is five generations removed from the Most Recent Common Ancestors (MRCA), a shared set of those 2G grandparents. One in every ten 3Cs don't share ANY common segments with each other. And because siblings get 50% of their DNA from each parent, but not the SAME DNA (other than identical twins), it is quite possible to match some siblings in a family of 3Cs but not every sibling.
Similarly, a 2nd cousin once removed (2C1R) is also five generations distant from that common ancestor (their 2G grandparents), and may not have inherited any of the same DNA from them as you did in scientifically significant amounts. But if we go a couple generations more distant - 5th cousins (5Cs) sharing a set of 4x great grandparents (4Gs) - and that number jumps dramatically. Nearly 9 of every 10 5Cs don't share any common segments!
This illustrates why a validated paper trail of records and research must be used along with DNA evidence. In general the farther into the past we go, the less certainty we can have about our findings. For the casual or serious family historian, to have all or most of the evidence we've accumulated support our hypothesis about a 4x great grandparent (4G) is usually sufficient. To a genealogist, a Standard of Proof is a very high level of certainty that is hard to attain with more distant ancestors. We'll explore these more distant relationships in the next post.
A few odd and important tidbits:
• By itself, you can't tell if a match is maternal or paternal; this can only be determined by comparison to known cousins.
• DNA doesn't lie, but people do. Sometimes things happen, not all of them good or pleasant. There are lots of possible explanations when the DNA does not support the tree or the family history that's been handed down. Consider them carefully, and gently.
• DNA doesn't "skip a generation"; if your parents didn't pass it on to you, you don't have it.
• Some segments of DNA pass through many generations unbroken, and originate too far into the past to be able to identify a common ancestor.
• The different testing companies use different algorithms to evaluate your DNA, so for instance the amount you have in common with the same DNA Match can vary considerably from one company to the next.
• While the practical threshold for an actual match varies typically between 7 and 10 cM (depending on which expert you ask), it is usually difficult to identify a Common Ancestor with matches of less than 20 cM. (Once in awhile you get lucky and both trees reach far enough back in the right branch.)
• We also can't determine which person - you or your Match - is closer to the Common Ancestor from the DNA amount alone.
• Lots of other relationships are possible - half relations for instance. And it wasn't that long ago that many people lived in small communities and routinely intermarried amongst families (2nd cousins marrying was quite common in the early 1800s). The practice of marrying exclusively within one's own ethnic, cultural, social, religious or tribal community is known as endogamy, and it is common in colonial America. If you have colonial ancestry it is quite possible to share several distant common ancestors with a single Match.
NEXT: They Have the Same 5xGreat Grandfather in Their Tree - We're a Match!!!
We don't inherit the same amounts of DNA from each of our 16 great-great grandparents (2Gs). As we go farther back through successive generations, we find that we don't have any identifiable DNA from many different lines in our family, while DNA from a few branches of our family may still show up 10 to 12 generations distant.
Autosomal DNA has been found to be reliable over 5 to 7 generations. A 3rd cousin (3C) is five generations removed from the Most Recent Common Ancestors (MRCA), a shared set of those 2G grandparents. One in every ten 3Cs don't share ANY common segments with each other. And because siblings get 50% of their DNA from each parent, but not the SAME DNA (other than identical twins), it is quite possible to match some siblings in a family of 3Cs but not every sibling.
Similarly, a 2nd cousin once removed (2C1R) is also five generations distant from that common ancestor (their 2G grandparents), and may not have inherited any of the same DNA from them as you did in scientifically significant amounts. But if we go a couple generations more distant - 5th cousins (5Cs) sharing a set of 4x great grandparents (4Gs) - and that number jumps dramatically. Nearly 9 of every 10 5Cs don't share any common segments!
This illustrates why a validated paper trail of records and research must be used along with DNA evidence. In general the farther into the past we go, the less certainty we can have about our findings. For the casual or serious family historian, to have all or most of the evidence we've accumulated support our hypothesis about a 4x great grandparent (4G) is usually sufficient. To a genealogist, a Standard of Proof is a very high level of certainty that is hard to attain with more distant ancestors. We'll explore these more distant relationships in the next post.
A few odd and important tidbits:
• By itself, you can't tell if a match is maternal or paternal; this can only be determined by comparison to known cousins.
• DNA doesn't lie, but people do. Sometimes things happen, not all of them good or pleasant. There are lots of possible explanations when the DNA does not support the tree or the family history that's been handed down. Consider them carefully, and gently.
• DNA doesn't "skip a generation"; if your parents didn't pass it on to you, you don't have it.
• Some segments of DNA pass through many generations unbroken, and originate too far into the past to be able to identify a common ancestor.
• The different testing companies use different algorithms to evaluate your DNA, so for instance the amount you have in common with the same DNA Match can vary considerably from one company to the next.
• While the practical threshold for an actual match varies typically between 7 and 10 cM (depending on which expert you ask), it is usually difficult to identify a Common Ancestor with matches of less than 20 cM. (Once in awhile you get lucky and both trees reach far enough back in the right branch.)
• We also can't determine which person - you or your Match - is closer to the Common Ancestor from the DNA amount alone.
• Lots of other relationships are possible - half relations for instance. And it wasn't that long ago that many people lived in small communities and routinely intermarried amongst families (2nd cousins marrying was quite common in the early 1800s). The practice of marrying exclusively within one's own ethnic, cultural, social, religious or tribal community is known as endogamy, and it is common in colonial America. If you have colonial ancestry it is quite possible to share several distant common ancestors with a single Match.
NEXT: They Have the Same 5xGreat Grandfather in Their Tree - We're a Match!!!
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