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Genes-R-Us —Kin, cousin marriages and the ‘founder effect’

Genes-R-Us —Kin, cousin marriages and the ‘founder effect’

Posted: Wednesday, March 7, 2012 5:00 pm

Some months back I discovered that a former husband was my fifth or sixth cousin. That discovery was made by using snippets of DNA data from a direct-to-consumer (DTC) genetic testing company for three individuals — my son, a third cousin and me. (“Pierre,” that third cousin, was identified through the Relative Finder feature of the DTC genetic testing company.)
Recently, I discovered yet another cousin, “Barthélemy,” who was a classmate of mine from the small town in Washington State where we both grew up. I now know that these three cousin relationships are exceptional, in that they embody what geneticists call the founder effect.
The founder effect is what happens genetically when a small, isolated group breaks off from a larger, more genetically diverse group and begins interbreeding. The genetic diversity of the smaller group decreases as cousins marry cousins.
For reference, it was only after the American Civil War that marriage between first cousins became illegal in any state. Most states still allow first-cousin marriage, including Tennessee, Alabama, Georgia, Florida, South Carolina, and Virginia; Louisiana does not, which is no accident.
In many Middle Eastern cultures, first and second cousin marriage is encouraged. Most of the world allows first cousin marriage except for parts of the U.S., China, India and certain parts of Eastern Europe.
Charles Darwin, Mr. Natural Selection himself, married his first cousin, Emma. Together they brought 10 children into the world with less genetic diversity than you might expect from Charles.
Scientists have observed the founder effect in animal populations on islands but it also happened at certain times in the settling of North America. The Amish are a great example of a group with a strong founder effect.
There is another very important group, with a strong founder effect that I have come to know as part of my heritage — the Acadians.
The Acadians came from France to settle in Canada in the 1600s, primarily in the eastern maritime provinces of New Brunswick, Nova Scotia and Prince Edward Island.
Because the Acadians had their own language (French), religion (Roman Catholic), and distinctive culture, they tended to marry within their own group for several hundreds of years.
They were skilled in draining water from the land to render it farm-worthy, first on the Canadian coast and later in Louisiana after they were all kicked out of Canada by the English during the Great Expulsion of 1755-1763. The sad story of the Acadian deportation was the inspiration of Longfellow’s Evangeline.
Once in Louisiana, the Acadians were called Cajun. My paternal grandmother descended from a line of Acadians and Cajuns that left a strong signal in my birth father’s DNA.
A high degree of consanguinity between procreating partners is at the root of the founder effect. Consanguinity is the amount of identical DNA shared between any two people and it decreases by a factor of four for each generation one goes out. For example, the consanguinity between two first cousins is 16 times greater than that between two third cousins.
Rare versions of genes (alleles) can take on a surprisingly high frequency in isolated groups, in just a handful of generations, due to the founder effect. Consanguinity increases the chances of offspring being affected by deleterious recessive genetic traits.
The Amish and the Acadians/Cajuns are considered a gold mine for genetic research due to the founder effect.
In these populations, an individual may appear to be healthy but he has a much increased likelihood of carrying carry alleles for deleterious recessive traits, like cystic fibrosis. Choosing a mate from the same population group increases the likelihood of that trait manifesting in their offspring.
My birth father was clueless that he was a carrier for cystic fibrosis (CF). He carries the defective version of the CF gene that is characteristic of the founder effect in French Canadian populations.
My father also has Crohn’s disease, an autoimmune disease, which is more frequent in populations with a strong founder effect like the Ashkenazi Jews, Acadians/Cajuns, and Amish.
Last fall, I wondered if I might be related to “Barthélemy” when I discovered that I had a “” French-Canadian cousin with his last name. Social networking made it very easy to contact my classmate, “Barthélemy.” We soon realized that our fathers both had Acadian roots.
“Barthélemy” needed little convincing to send a sample to 23andme for DNA testing.
However, his sample never made it to lab, he discovered after months of waiting in vain for his results. He has now submitted a new sample.
In the meantime, I started genealogical research on his family tree, dabbling in it primarily when my house needed cleaning. Finally I started bumping into last names on his family tree that I recognized from my family tree (although both trees are very incomplete).
I struck gold on Sunday. I identified our first common great (times seven) grandparents on Great-times-seven Grandpa was one of the original 31 French farmers arriving in New France on May 1654. He married Great-times-seven Grandma a couple years later. They had at least eighteen children and 159 Grandchildren.
Now that I understand more of the Acadian history, marriage, and childbearing patterns, I look forward to comparing our DNA segments once “Barthélemy” has his data. Given the founder effect in the Acadians, I expect that we may find that we are double, triple, or even more closely related kin.
By the way, did you know that “kin” is Japanese for “gold”?
Nancy Miller Latimer has worked in scientific research and development for 27 years. She blogs at and may be contacted by email at Nancy@ Published in The Messenger 3.7.12


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