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Consanguinity conundrum

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GuyBarry
1249985.  Thu Sep 21, 2017 5:10 am Reply with quote

This popped into my head this morning as I was listening to a radio report on something else.

In the UK, it's illegal to marry one's sibling, but legal to marry one's first cousin. (There are arguments about whether it should be legal to do so, of course; but put those to one side for a moment.)

Suppose you had identical twin sisters who married identical twin brothers. Then the children of each marriage would be first cousins, and so legally entitled to marry each other. Yet they would share the same amount of genetic material as if they were siblings.

How can this be justified?

 
ali
1249986.  Thu Sep 21, 2017 5:23 am Reply with quote

It's a good question. There has always (at least, since 1560) been a lot of weirdness in the restrictions, less now than there was, but still some.

Here is some info on the subject.

 
PDR
1249992.  Thu Sep 21, 2017 6:39 am Reply with quote

GuyBarry wrote:
How can this be justified?


It probably can't. The law can't hope to cover every case ("hard cases make bad law") and the real significance of identical twins only become apparent ong after the bulk of the incest law was framed anyway.

Most incest law is based on "common law" (the accumulated body of recorded judgements going back over a thousand years excluding the period 30 January 1649 to 28 May 1660 inclusive) rather than evidence-based statute. There is is general initiative to tidy this up by reviewing/deleting/updating/replacing it with more considered legislation (this is the buld of the parliamentary day-job) and it wouldn't surprise e if incest law was in the list somewhere. But your "pairs of identical twins" case is a rare occurance so I suspect it's not near the top of the list.

PDR

 
jaygeemack
1250272.  Sat Sep 23, 2017 5:54 am Reply with quote

A study in Bradford a few years ago found that the occurrence of birth defects in children in the city was about twice the national average. In the Pakistani community there, 37% of children had parents who were first cousins.

 
suze
1250320.  Sat Sep 23, 2017 12:49 pm Reply with quote

Professor Steve Jones of UCL and the former MP Phil Woolas (Lab, Oldham East and Saddleworth) have both tried to raise this issue, but the Muslim Public Affairs Council denounced both as racists and told them to mind their own business.

If that is how the Pakistani community in Bradford is going to react, is there any hope at all of the issue being addressed?

 
crissdee
1250321.  Sat Sep 23, 2017 12:55 pm Reply with quote

I probably mentioned this before, but I read an article many years ago in "Bizarre" magazine which claimed that birth defects linked to cosanguinity were getting far more prevalent in the vast housing/community developements such as Byker in Newcastle(?) because fewer people were travelling out of the area for work or play. The estates provided almost everything they needed (including sex partners) so the gene pool had shrunk to the level of native jungle communities.

 
Strawberry
1250347.  Sat Sep 23, 2017 4:27 pm Reply with quote

crissdee: Yes, you have mentioned it before but not recently. :P

Post 310883

 
jaygeemack
1390413.  Mon Sep 20, 2021 4:41 pm Reply with quote

There is a story in today’s Mailonline (yes, I know I shouldn’t) about identical twins dating identical twins in USA.
[url] https://www.dailymail.co.uk/femail/article-10009465/Identical-twin-sisters-dating-identical-twin-brothers-LIVE-share-75-dates.html[/url]

 
tetsabb
1390420.  Mon Sep 20, 2021 6:29 pm Reply with quote

One day many years ago I saw two couples in our town square -- identical brothers with identical sisters, identical cars and caravans.

 
CB27
1390454.  Tue Sep 21, 2021 6:31 am Reply with quote

GuyBarry wrote:
Suppose you had identical twin sisters who married identical twin brothers. Then the children of each marriage would be first cousins, and so legally entitled to marry each other. Yet they would share the same amount of genetic material as if they were siblings.


Have to pick up on this.

Typical siblings are reported to share 50% of their genes.

Typical half siblings are reported to share 25% of their genes.

Typical first cousins are reported to share 12.5% of their genes.

Mathematically speaking, it can be presumed that the offspring of identical twins (monozygotic) who each marry the other twin will share 50% of their genes, because their parents have a 100% match.

Except that's not really true.

Monozygotic twins share the same genes when the embryo is created, but as the embryo divides into two, the development of their genes will vary by other conditions, and minor mutations mean they have slightly less than a 100% gene match.

This is true of all children. This is why siblings in reality tend to share slightly less than 50% of their genes and so on.

The next level of children then inherit their genes differently (they won't be genetically identical to each other), and their genes may express themselves in different ways according to the conditions of their birth and development, as well as any minor mutations in their embryonic development.

So, while a DNA test might reveal that these offspring are matching at significantly higher than 25% and may identify as siblings, they've gone through two generations of slightly reduced matches that could result in a slightly lower match than your average siblings.

 
CB27
1390457.  Tue Sep 21, 2021 6:53 am Reply with quote

I have to admit, maybe because I was close to most of my cousins, I find the idea of relationship with cousins (or even siblings) a bit creepy, but I'm also aware of how prevalent it was in all our history, and it led to our survival growth as a species, so we need to study it more objectively.

With relation to the comments about birth defects and consanguinity, it should be noted that in many studies, including the one in Bradford, the rate of defects, while higher than the rest o society, was still very small.

In the Bradford data, the rate was 6% of births among related parents compared to 3% where they weren't. It was also noted that in some of these cases the mother was older than the average, and the study revealed that regardless of relation of parents, older mothers carried a higher risk of birth defects (though still a small number).

Obviously most people would like to reduce the chances of any birth defects, so avoiding consanguinity is preferable, but there there are other methods.

For example, among very Orthodox Jewish societies, it's not simply a matter of couples being related immediately to one another, it's the fact that population bottlenecks in the past (especially across Eastern Europe) have caused several generations of interbreeding that led to some genetic conditions growing stronger.

There are services, like Dor Yeshorim, which let Orthodox Jews register a DNA sample (often as kids), which will then be checked when couples consider getting married and starting a family. These tests will allow them to check if there are any genetic markers in both parents that could increase the potential for either birth defects or passing on genetic disorders, rather than avoid marrying altogether.

 
jaygeemack
1390487.  Tue Sep 21, 2021 8:59 am Reply with quote

Iceland has a small gene pool, and they have a dating app which allows you to establish how closely related you are to a potential partner.

 
PDR
1390488.  Tue Sep 21, 2021 9:03 am Reply with quote

jaygeemack wrote:
Iceland has a small gene pool, and they have a dating app which allows you to establish how closely related you are to a potential partner.


Tennessee and Mississippi have similar apps so that people can confirm that prospective partners are 2nd cousins or closer...

PDR

 
jaygeemack
1390504.  Tue Sep 21, 2021 10:25 am Reply with quote

I have friends who are twins, but only one of them is identical.

 
cornixt
1390506.  Tue Sep 21, 2021 10:27 am Reply with quote

CB27 wrote:
There are services, like Dor Yeshorim, which let Orthodox Jews register a DNA sample (often as kids), which will then be checked when couples consider getting married and starting a family. These tests will allow them to check if there are any genetic markers in both parents that could increase the potential for either birth defects or passing on genetic disorders, rather than avoid marrying altogether.

Didn't it used to be common for people to have blood tests before marriage in the USA? I remember several American shows in the 80s where it was a major plot point that something terrible would be discovered due to one.

 

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