Endogamy or Pedigree Collapse, and Genetic Genealogy

How much faith can you place in the degree of relationship predictions at FTDNA, 23andMe and AncestryDNA?

Below is a chart produced by the Exploring Family Trees program at https://learnforeverlearn/ancestors/ website from a GEDCOM of my direct ancestors. My maternal ancestry is on the left and my paternal ancestry is on the right of the chart. Click on the chart to enlarge it.

The horizontal lines going from one side to the other side show the ancestors shared by my parents and also show that my mother's parents had shared ancestry. This is due to cousins marrying cousins and is known as Endogamy. Most people will show it in their own ancestry. It is also called Pedigree Collapse since the number of distinct ancestors at a certain generation will be less if you have more than one line of ancestry from an ancestor or ancestral couple. In my case it is considered to be a mild instance. Indeed, when I run my FTDNA autosomal result file through the program by David Pike to determine Runs of Homozygosity, I have none.

However, when I look at my 1st, 2nd, and 3rd cousins, the intermarriages in my ancestry combined with endogamy within the set of people available for marriage in the area of southeast Kentucky from 1800 to 1940, results in my genetic family tree having an abundance of cousins who are related to me on multiple lines, and often being shown as having a closer degree of relationship that that expected based on traditional relationship charts.

Comparing the relationship predictions at the three main genetic genealogy testing companies, people are predicted to be one-half or one full-step closer at both FTDNA and 23andMe, and one-half to one full step farther away at AncestryDNA. Two of my four tested full 1st cousins exceed the expected 12.5% of DNA sharing: one is 17.9% (we share 6 of 8 great grandparents) and the other is 15.9%. The other two, a brother and sister, are at the expected 12.5%. Due to the Timber algorithm used at AncestryDNA I have a full 2nd cousin who is a predicted 3rd cousin at Ancestry. Her brother is shown as a full 2nd cousin at FTDNA.

FTDNA, through its pedigree system lets you set the exact relationship with your DNA matches. Neither 23andMe nor AncestryDNA do so. We need to be aware of these differences when checking predicted relationship at the different company sites.

Multiple cousin relationships and shared DNA: Endogamy and Consanguinity

I have read a number of posts about endogamy and shared DNA in regard
to the Ashkenazi "Jewish correction" used by 23andme and how it keeps
Ashkenazi Jews from seeing their non-Jewish cousins. Because of the
many intermarriages among European Jews the genetic relationship
predicted is often greater than that expected from the documented
genealogical relationship. These predicted higher or closer relations
push out the genetically smaller but documented higher genealogy
relations.

Many of the Ashkenazi cannot determine the relationships being shown
since the predicted range is too close and their pedigrees are not
detailed enough to make the connection. Below I'll give a non-Jewish
endogamy example where the relationships can be determined.

Last night I received a predicted 2nd cousin at 23andme.  The per cent
of shared DNA is 3.02 in 13 segments for 226cM. He (call him A) is
actually one of my multiple cousins. We (I am B) are 3rd cousins, 3rd
cousins once removed, half 3rd cousins once removed, 4th cousins, 5th
cousins, and triple 6th cousins.

Compared to my multiple 1st cousin (C) they share 10 segments for 151cM.

Compared to my and my 1st cousin's maternal double 3rd cousin once
removed (D) they share 1 segment of 0.9cM.

A is related to B and C on both maternal and paternal lines and to D
through his paternal grandmother.
B is related to A and C on both maternal and paternal lines and to D
on 2 maternal lines.
C is related to A and B on both maternal and paternal lines and to D
on 2 maternal lines.
D is related to A on one maternal line and to B and C on 2 maternal lines.

In all of these cases the predicted relationship, based on DNA shared,
was one generation closer than the documented relationship mainly due
to the high degree of endogamy and the resulting consanguinity.

Now I am waiting for A's wife's results, she is my 3rd cousin once
removed and, yes, we share multiple lines of ancestry. ;^)