Double First Cousins

Recently, someone on an Appalachian oriented Facebook group to which I belong and on which I often post, brought up what they called "double cousins".  In my experience, the appropriate term for such relatives is "double first cousins" since there can be no such thing as "double cousins" in any other generation.  If you look at a genogram for double first cousins, it is readily apparent that because their parents, all four, are composed of two sets of siblings with two sets of parents in common.  This gives these two couples' children a common set of four grandparents.  But if you try to create a situation with "double cousins" in the next generation or any subsequent generation, it is impossible.  And, therefore, "double cousins" is a misnomer.  When the double first cousins marry outside the family of origin, the closeness of the DNA relationship becomes more distant.  We all inherit roughly 50% of our DNA from each parent on a totally random basis, roughly 25% of our DNA from each grandparent also on a totally random basis.  Only identical twins conceived by one sperm cell which impregnates one egg and that fertilized egg spontaneously divides into two fetuses shortly after fertilization.  Each of us had 46 chromosomes divided into 23 pairs.  Each of those chromosomes contain a total of 20,000 to 22,000 genes, the majority of which have two or more alleles.  That means that each human has roughly 1,012,000 alleles which means that it is nearly impossible for even the best mathematicians to calculate the possibility that any two double first cousins could have inherited identical genetic material in the totally random manner in which it occurs. 

I grew up with two sets of double first cousins.  In the first case, one of my maternal uncles (A) married a woman (B)  from a particular family and they had five children (Y) together.  Then, one of my maternal aunts (C) had an illegitimate son by a brother (D) of the aforementioned uncle's wife (B) .  Those six children (X)  are all double first cousins.  In the second case a son and daughter (Y)  of the aforementioned maternal uncle married a son and daughter (Z) from a third family and had about 8 or 9 children (XX) , all of whom are double first cousins.  While it is very remotely possible that the grandchildren of my uncle who are double first cousins, as are his own children, are nearly identically related, those grandchildren are not much more closely related to each other genetically than if their parents, the original double first cousins had married two people who were totally unrelated.  Now to make this issue even more confusing, another of my maternal aunts (E) , the youngest in fact, married a son (F)  of one of the sisters of the woman my uncle married and to the man my aunt had an illegitimate son by and that youngest aunt and her second generation husband had two children (YY).  That makes for some very complex genograms but it does not make those last two children much more closely related to the first twelve or thirteen than if they were only first or even second cousins.  You sort it out!  

But, in order to actually learn something about this, let us look at some of the sociological reasons siblings marry other siblings and create first cousins.  In almost all cultures, including Appalachia, where double first cousins are common, those cultures are frequently either located in geographically isolated areas or they practice a relatively small and strict form of religion.  In the case of Appalachia, those two reasons are both found although geographic isolation is now less common that it was 50 years and beyond in the past.  But relatively small and strict forms of religion are still practiced in much of Southern and Central Appalachia.  A few examples are serpent handlers and other Pentecostal and Holiness churches, Old Regular Baptist, and Primitive Baptists, and a few other minor sects based on uncommonly held stringent beliefs such as Jesus Only, etc.   Nearly all of these religions produce children who generally marry inside the church in which they were raised.  

Now let's look at what is called "consanguinity" which is a term for being related to another person by blood.  When people with a consanguine relationship marry and produce children, it is much more likely that some small percentage of those children will be born with some form of genetic disease or flaw.  Several groups, both based on religion and/or geography have one or more forms of genetic disease or flaws.  In Appalachia, the best known such group of people are the Blue Fugates of Eastern Kentucky who had a genetic condition called methemoglobinemia, which was passed down through a recessive gene and blossomed through intermarriage.  Phenylketonuria, a condition which causes persons who are born with normal intelligence to slowly become mentally disabled if they ingest phenylketones in their food.  Cola drinks today carry warnings about the dangers of phenylketonuria which is most common among people who were either born in or descended from natives of Northern Ireland, another region where geography impaired outside marriage and made consanguinous marriages more common.  Ashkenazi Jews are likely to inherit nearly a dozen genetic based diseases which arise due to their frequent and long term history of intermarriage within their religion.  So do the Amish and some conservative Mennonites and I have several friends among both those religions.  The Uyghur populations of Northern Iraq, who practice a unique form of Christianity, have a very high rate of diabetes due to intermarriage.  

In my practice in the human services professions, I knew one small boy who had inherited phenylketonuria although I do not know a lot about his genetic history.  I have also known numerous Amish and Mennonite people and have long been aware of their propensity to inherit polydactyly, extra fingers or twos, along with the large number of genetic diseases they can inherit.  I also met one couple in Eastern Kentucky who had chosen to marry despite being first cousins.  But they were very intelligent and moralistic people and chose, before marriage, to come to an agreement to never have biological children and to adopt which they did due to a relative who lost custody of two children due to a history of abuse and neglect of those children.  So what have I learned from all this which I wish to pass on to my readers?  No one should consider engaging in a consanguinous marriage!  Period, The End!