RootsChat.Com
General => The Common Room => Topic started by: Mart 'n' Al on Monday 30 July 18 14:53 BST (UK)
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I was going to ask a question about the respective genetic dangers of marrying a first cousin as opposed to a second cousin. I thought that if marrying a first cousin was so dangerous, why are we allowed to marry a second cousin, only to find out that it's not illegal and possibly not dangerous.
Before starting to write this comment, I did a bit of research and found that much of what I believed was true about cousin marriages is actually incorrect. I won't repeat everything that I've learnt here today, but two interesting facts are, firstly, 10% of all marriages worldwide are between first cousins, and, secondly, one particular unspecified religion used to prohibit any marriage closer than 6th cousin. That must have taken a lot of research, soul searching and family history management before going ahead with the relationship.
I won't ask my question about the genetic dangers, as from what I've read this afternoon these seem to be still unproven if not totally apocryphal. I'm sure I will get a lot of response to this, and strongly recommend that you do a bit of reading in advance. I was very surprised by what I've learned.
Martin
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There is a long list of famous first cousins marrying! It's far more common than a lot of people believe.
See: https://en.wikipedia.org/wiki/List_of_coupled_cousins
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The problem isn't with an occasional cousin marriage; the very real genetic issues arise as a result of widespread cousin marriages occurring throughout a population repeatedly through many generations over hundreds (or more) years.
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Also, when cousins marry, there's a higher chance of recessive gene problems in the offspring.
OH has haemochromatosis, which means that he doesn't get rid of excess iron - for most versions of this it's caused by the sufferer receiving a faulty gene from both parents. If the parents are closely related, it's somewhat more likely that both may be carriers of the same recessive genes.
Because of OH's haemochromatosis, all our children are carriers of the gene, though none are sufferers. If one of them married a carrier, statistically one quarter of their children would be sufferers. It's a fairly common gene in Ireland anyway.
(no first cousins involved - OH is an "only")
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My grandparents were 1st cousins,their dad's were the 2 brothers nearest in age to each other.
My dad was an only child (as am I) and we're all okay!!!
The problem as someone has said is when lots of generations marry into succesive families and offspring become in danger if inheriting gene problems from both sides of the same families.
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This subject has come up many times on RootsChat, and there are many posts, just put "cousins marrying" in Search.
As is well known, in 1540 parliament legalized marriages between first cousins to clear the way for the marriage of Henry VIII to Catherine Howard, although nowhere did the Bible indicate that cousin marriage was undesirable. What is perhaps not so well known is that in 1870, Darwin's close associate, John Lubbock, was elected to parliament and Darwin urged Lubbock to propose the inclusion of a question on cousin marriage in the 1871 census. It would then be established whether families in which the spouses were cousins had fewer children than the average. If so, 'we might safely infer either lessened fertility in the parents, or which is more probable, lessened vitality in the offspring.' Later it might also be possible to find out whether or not 'consanguineous marriages lead to deafness, and dumbness, blindness, etc.' The proposed census question was rejected by the House, although, in committee, the vote was forty five for and ninety two against. George Darwin, Charles's eldest son, then carried out a detailed study and found that there was no evidence that such marriages had significant deleterious consequences from the biological point of view.
[George H. Darwin, 'Note on the Marriage of First Cousins', Jl. Statistical Soc., xxxviii (1875)]
Stan
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In the area of North West Essex (Saffron Walden, Finchingfield etc) I have noticed a number of first cousin/second/third cousin marriages. I think 3rd cousins and 4th (and 5th and so on) cousins marrying is more of a no brainer, and quite common if the families of the spouses lived in the village/surrounds for centuries.
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Again, it's not a problem if it happens occasionally in a gene pool. But cousins who are the children of cousins who were the children of cousins, and so on, going back hundreds of years... well, that does increase the likelihood of certain genetic defects. For example:
https://www.bbc.com/news/uk-england-leeds-23183102
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See this article.
http://www.genetics.edu.au/publications-and-resources/facts-sheets/fact-sheet-18-when-parents-are-relatives-consanguinity
To quote :
"If parents are unrelated, their chance of having a child with a birth defect or disability is between 2% and 3%.
If parents are first cousins, the chance is a little higher at 5% to 6%. This is due to the increased chance that they will both carry the same autosomal recessive mutation, passed down through the family."
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I have first cousins marrying whose children had issues. However in addition to their father's being brothers their mothers were first cousins of each other and their paternal grandfather and grandmother were second cousins of each other.
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See this article.
http://www.genetics.edu.au/publications-and-resources/facts-sheets/fact-sheet-18-when-parents-are-relatives-consanguinity
To quote :
"If parents are unrelated, their chance of having a child with a birth defect or disability is between 2% and 3%.
If parents are first cousins, the chance is a little higher at 5% to 6%. This is due to the increased chance that they will both carry the same autosomal recessive mutation, passed down through the family."
My question would be whether a doubling or tripling of the chances can be accurately described as being "a little higher."
Yes, overall, 6% is a lot less than 94%, but 6% is three times the rate of 2%.
And then, are we dealing with 6% of 100 people or 100,000 people? If the latter, the number of potentially affected children would rise dramatically.
(I know you're not the originator of the quote; my comment is not directed at you but more in general.)
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I also have a few instances of first cousin marriages & at first thought this was unlawful, Google to the rescue!
I do get the impression from two of my examples that the practice was not approved by the local community/families. One couple chose to marry in the nearby town rather than the local church where siblings had married and almost immediately moved to Scotland. Another I was told had caused upset in the family on her marriage and also had moved away from the home village. This may have had more to do with her widowed father & four brothers losing their free housekeeper though!
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You might want to take a look at the history and the genealogy of isolated settlements in North America. You will find that marriage to a first cousin was inevitable in remote communities unless the child of the first generation of settlers abandoned family investment and took to the road in order to explore opportunity in another community at further remove.
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As mentioned before one marriage between cousins will only increase health problems a little but if cousins marriages continues through several generations there will be greater problems.
The link below includes a family tee for King Charles II of Spain and shows what happens when royalty are restricted in who they can marry
http://www.genealogyintime.com/NewsStories/2009/April/inbreeding-of-spanish-royalty-page1.html