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.
