Endogamy - is a word used to describe a genetic phenomena that occurs when people live in limited communities. They are limited in choice of sexual partner by religion, geography or lack of people in the community. We are probably most familiar with inbreeding in domestic pets and the problems it can cause. Great care was taken in the past to ensure that there were enough degrees of separation in prospective parents – degrees of sanguinuity?
Sanguinuity is the blood relationship in a family and Affinity would be the spouses 'sanguinuity'. Using this system it was possible for someone who was a relative of both parties to be identified. It is one of the reasons for Banns. The public has a chance to say why they think the marriage should not be allowed.
DNA and its transmission from one generation to another is very cleverly designed to be constantly changing and mixing. The chromosomes themselves are always breaking and reforming, sometimes in subtly different ways - parts of your parents paternal and maternal DNA can switch places in their chromosomes. Together these are the reasons why your DNA connections fade over 6-8 generations.
However, when a community lacks choice of sexual partner from a different community the genes instead of spreading out become retained in the community and endlessly recycled.
There are different degrees of Endogamy.
No Endogamy - your parents are not related - all your ancestors are unrelated
Pedigree Collapse - one or more of your ancestors were related to the lines that produced your your mother and father
Endogamy - multiple relatives and ancestors in common between your mothers line and your fathers line
Severe Endogamy - very many or all of your mother and father’s ancestors are related
In Irish Roman Catholic Baptism registers you will see “degrees of consanguinuity” written usually with something like 3rd and 4th following. All ancient communities were aware of the genetic dangers of inbreeding and took steps to prevent it - the reason why close cousins could not marry and why we have the expression “kissing cousins”. It was important to be three and ideally four generations past a common ancestor before cousins could marry. Sometimes you will read the licence is granted by the archbishop, an appeal will have gone above the local priest to allow the marriage. (See the DNA cousins chart for how little 'common DNA' is likely at this level).
Jewish communities are the famed text book group who intermarry from a limited pool and they take great care about the generation distance between the ancestors of a couple looking to get married. Finland is another population apparently and Ireland too. However, it can just as easily be small rural villages or isolated places like islands that can have the same problem. A well documented text book case is an alpine village in Italy gives 4 family names and hundreds of people with the four names. I personally know of a village in Spain of 148 people where 126 are related by blood. Christmas in Spain is normally an open door event for relatives anyway but here it is for the whole village!
In Ireland one of the phenomena I have seen is what I term the Chromosome 6 Anomaly. (not very snappy I know!). Chasing many people with geographic links to the mid Tipperary area and its border with Limerick, I can find good geographical connection, the next townland / village, timeline, common names but no DNA match. Drat! However, wait, what is this in my GedMatch result?
Chromosome 6 Anomaly - Endogamy visualised
These two large patches of yellow and green occurred in at least half of these tests of suspected cousins. At first I was puzzled then, after dozens of examples, I knew it was a definite effect.The DNA match is on the extreme right, yet the patches in the regions 20% to 45% from the left side are not matches!
It can occur with and without accepted DNA matches. It is caused by the constant mixing of genes and the DNA being so mixed, again and again that there are thousands of small DNA fragments in common and you would not know if they were Ryan, Dwyer, Henessy, Murphy, Leary or whatever other names are in the area. Even today this is a very rural area of Ireland but at least now there are more options for potential parents.
Another of Endogamy's visible effects is to see a good % or good cM match but multiple small or very small DNA matches scattered across different chromosomes.
.Seamus Crowe Website2021