Endogamy

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 family distance in prospective parents, degrees of affinity and 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 Marriage Banns. The public have a chance to say that the parties may be related. amongst other impediments.

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 (recombination). 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 mother and father

Endogamy - multiple relatives and ancestors in common between your mothers line and your fathers line. Probably other families in the location also have endogamy.

Severe Endogamy - very many or all of your mother and father’s ancestors are related

In Irish Roman Catholic Baptism registers, sometimes 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 cousin's'. 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 inter-marry 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 Arcadia (Newfoundland, Canada), Ireland too. However, it can just as easily be small rural villages or isolated places like islands or mountain valleys that can have the same problem. A well documented text book case is an alpine village in Italy. It 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 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. However, 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 seem larger but are not matches!

UPDATE: I have discovered this area is only 4cM and that is why cM does not relate to length and why this result is not included in matches, as it is below the 7cM limit,

It can occur with and without accepted DNA matches. It can be caused by the constant recycking of the same DNA and probably this is a region where DNA does not break and re-combine very easily, meaning it would stay the same for more generations.

Another of Endogamy's visible effects is to see a good % total or good cM total match but made up of multiple small or very small DNA matches scattered across different chromosomes.

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