Tip 18: Pop Quiz (Statistics)

I struggle with statistics as I am sure some others do. It can get hard to wrap our heads around all the numbers involved with DNA. If you don’t know it yet, ISOGG is the International Society of Genetic Genealogists. It is pretty much the last word in the science of what we do. This page gives you the actual information about how much various relatives should share with you. I like to go back to this.
https://isogg.org/wiki/Autosomal_DNA_statistics
I have a message from someone I helped a long time ago to find his father. He is now very good at what he does. However, I sometimes get odd little queries from him.
His Email read:
This is not very scientific and I am not sure what it means. However the other day I noticed that some of my distant matches shared more cM than predicted. So I made this table using the data on https://isogg.org/wiki/Autosomal_DNA_statistics. I found the results interesting and thought to share.
What I noticed in the statistics is that the theoretical size of the match from ISOGG predictions is much lower than the actual size of the matches. I am not sure what this means.
So this chart compares theoretical Average of these relatives with the observed average of these relatives. The really interesting part is the bottom half.
He had some theories as to why this happens but I think I have additional reasons.
I thought maybe you would enjoy thinking about it. Why do you suppose the real relatives are so far off the mark? What should I write back to him? I have not checked his stats against what is on ISOGG. Maybe I should have. But they don’t show all those half and removed relationships. I seem to spend my life in those.
Tell me all the reasons you can think of. If this blows your mind, don’t worry about it. Just look below for the answer.
For your information, his mother was from England, and his father had Colonial American heritage.
June Byrne
DNAAdoption.org
PS also check https://dnapainter.com/tools/sharedcmv4

Answer:
The answer to why is endogamous populations. England and Ireland are islands. Those with a high % of early colonial family backgrounds have all sorts of 6-7-8 cousins. So what you are looking at is endogamy but it is so far back you can’t find it. If people stayed in VA for several generations in semi-rural counties, they ran out of non-relatives to marry. Boys married the sister of their sister’s husband etc.