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If you have endogamy in your family tree, have you ever wanted to know whether or not you have more or fewer false matches than other test takers?
Defining Genetic Genealogy terminology
When we're talking about endogamy and false matches, we need to define our terms before answering our main question.
Endogamy
You might have heard of endogamy, especially concerning certain populations like the Acadians, Ashkenazi Jews, or the Polynesians. Endogamy is the marriage of related persons within a group over several generations.
In a nutshell, you basically end up having the same families, all intermarrying over a long period of time.
Pedigree Collapse
Endogamy is not the same as pedigree collapse or tree collapse. Pedigree collapse means you are related to somebody in multiple ways. This happened when your relative or shared ancestor married within the same family. Brothers marrying sisters is a common example of tree collapse.
The key to having endogamy is pedigree collapse over several generations, in many cases hundreds of years. Not many people outside of the group marrying into the families.
Is St. Kevern, England endogamous?
To give you an example, I have ancestors from Saint Kevern, Cornwall, England. St, Kevern is a little town in the 1800s with about 2,500 people. It was a farming community, so the majority of the people (more than 90 %) of the people were born in St. Kevern and lived there for several hundred years. However, with 5-10% of new families moving to St. Kevern from neighboring towns and the seafarers, this community is not endogamous.
Intermarriages between different families happened regularly over a few generations, but we don't see it over several generations.
Why are the other groups endogamous?
The Ashkenazi Jews, for religious reasons, married within their group. They stayed a very tight-knit group, and therefore are endogamous.
With the Polynesian islands, only a small set of people hopped between each island. They intermarried, and their DNA is endogamous. As the islanders spread throughout the South Pacific, they settled like in Hawaii or New Zealand. As such, a much smaller amount of DNA spread throughout these islands and countries. This is why many Hawaiians and New Zealanders look like they could be second cousins even though their populations have been separated for hundreds of years.
What is a false match?
A false match is a DNA match on your list that it's not related within a researchable genealogical time frame. False matches happen because of chance. Or it could be because you are related, but everybody in that population has that same DNA. Thus, the common ancestor is beyond where any records will link you with your DNA match.
While we try to provide written versions of our videos, this is one time when you really need to watch the video to understand endogamy at a chromosomal level.
Endogamy and False Matches
In the video above, I use the whiteboard (a fan favorite) to draw out what happens at the chromosomal level when parents pass on their DNA to their children. The first draw focuses on DNA inheritance without endogamy.
I also explain how DNA testing companies can determine whether you received a DNA segment from your mother or your father.
A false match happens when a DNA match shares a segment with you that cannot be attributed to your father or mother. Now your match could be related to both of your parents.
But that segment of DNA has to have come down from your mother or your father.
When our DNA is analyzed, the DNA testing companies initially don't know which side of your chromosome comes from your father or mother. If you have your father or mother tested, some algorithms help separate your DNA matches into your paternal or maternal lines.
But there are always matches, particularly with smaller segments of perhaps 20 SNPs, that are false matches. Essentially, the smaller the shared segment, the more likelihood of a false match happens.
Again, in the video, I draw out how DNA inheritance occurs, but this time, I present an endogamous chromosome.
What you'll see is that determining whether a shared segment of DNA between you and a DNA match becomes very difficult. The algorithms might be able to determine whether a match is with your paternal or maternal line, but often a false match result is chosen because the lineage isn't clear.
In short,
some of your paternal matches could be false matches.
Some of your maternal matches could be false matches.
In endogamous populations, the DNA segments are so similar between the maternal and the paternal side that false matches are often called.
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How Many More False Matches Occur With Endogmay?
When we're talking about numbers, how many matches does a person with endogamous heritage have compared to somebody who does not?
Let's compare me with a friend, who knows they have an endogamous family.
I have approximately 30,000 matches on Ancestry.
My friend has 300,000 matches on Ancestry.
Within the past year, Ancestry has tried to limit such matches because of this problem with endogamy. But basically, he has 10 times the number of matches that I have.
When you look more closely, if I had a DNA match that shared 30 centimorgans, the segment would likely be on a single segment.
By contrast, when my friend with endogamous heritage has a DNA match sharing 30 centimorgans, they might have as many as four segments for 30 cMs to split across.
This is one way you can actually tell how endogamous your heritage is- If you see smaller segments that add up to larger amounts of shared cMs.
The big takeaway is that if you have endogamy in your family history, you will have many more matches, but many of them are false matches. You will not be able to identify a most recent common ancestor (link) because the lineage link does not happen within a genealogical researchable time frame.
Will you get more matches?
It depends on how you create your Superkit.
If you are using that 23andMe version 3 test as the baseline, then you're probably going to get maybe 10% percent fewer matches.
If you're using your Ancestry as the main kit with one other kit, then you might be getting significantly fewer matches.
With my experiments, I have seen that other than the Ancestry kit, both of my Superkits have fewer matches than any one of the other testing kits.
When it comes to whether or not you're going to get matches that are 20 centimorgans or more, what I find is there's actually very little difference between these Superkits.
So while you may get fewer matches, it doesn't look like you're going to get significantly fewer of those matches that are of the higher CM amounts.
To sum up, what you're doing with a GEDmatch Superkit is eliminating many false matches or matches that are marginal. Whereas, with DNA matches that share more than 20 centimorgans, those will be solidified.
Such matches are so long and have many SNPs involved, so adding more SNPs has a very low likelihood of turning that into "Not a Match" or removing that match from your list.
Hopefully, that answers your questions about how the GEDmatch Superkit affects the number of DNA matches you receive.
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