To think first-degree cousins should not marry?

[Onestonetogo]

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I can't help but wonder where you would have drawn the line exactly between a 'healthy' baby and an 'unhealthy' one.

Ok, figures-

If you are pregnant at the age I am (41), you have a 1:84 chance of having a baby with Down's (which is not fatal, nor especially life limiting except in very severe cases). Obviously there are other conditions that may affect your baby, but some are screened for and some are incredibly rare. So overall, in the absence of any other indications, you are most likely to have a healthy baby even at 41 (my baby's chance of Downs, adjusted, is 1:518)

If you and your partner carry an autosomal recessive disorder, every child has a 1:4 chance of having the disorder from birth. In the case of Jux's friend's, four of their children were not lucky in the draw. 2:4 of your children are most likely to carry the disease but not express it. In short, you have a 1:4 of having a totally healthy child. For every child. Your children might be unlucky: every single one might be affected. If you carried a child with a potentially fatal or very life-limiting condition with those odds, you'd surely want to know about it. It might even make you think twice about having children with each other. Surely?

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But maternal age is not about genes that you carry - it's about disorders that may occur anew entirely because your eggs are old. family history is irrelevant in assessing the risks for women over 40. There are a gazillion genetic defects which are more likely to occur in older eggs.

An amnio cannot guarentee a healthy baby to anyone.

On another tack - most carriers of recessive disorders do not know they are carriers and for many others there is no test available anyway.

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We were not able to freeze eggs 20 years ago or I might have done the same!

By duchesse Fri 06-Mar-09 16:17:10 Add a message | Report post | Contact poster

Ok, figures-

If you are pregnant at the age I am (41), you have a 1:84 chance of having a baby with Down's (which is not fatal, nor especially life limiting except in very severe cases). Obviously there are other conditions that may affect your baby, but some are screened for and some are incredibly rare. So overall, in the absence of any other indications, you are most likely to have a healthy baby even at 41 (my baby's chance of Downs, adjusted, is 1:518)

If you and your partner carry an autosomal recessive disorder, every child has a 1:4 chance of having the disorder from birth. In the case of Jux's friend's, four of their children were not lucky in the draw. 2:4 of your children are most likely to carry the disease but not express it. In short, you have a 1:4 of having a totally healthy child. For every child. Your children might be unlucky: every single one might be affected. If you carried a child with a potentially fatal or very life-limiting condition with those odds, you'd surely want to know about it. It might even make you think twice about having children with each other. Surely?

BUT a recessive disorder has to be carried by both parents in order to have 1:4 odds of ending up homozygous and therefore affected.

Even for cousins - assuming one of the grandparents are a carrier of the receive gene. There is only a 1:2 chance that each parent is a carrier...therefore 1:4 chance that BOTH parents of the cousins happen to be carriers. And then there is a 1:4 chance that a child of teh cousins is affected...

So when you follow all that down...you start with a single recessive carrier as a grandparent (of the cousin) and end up with a 1:16 chance that the offspring of marrying cousins has both recessive genes...which agreed is still higher odds of Downs to a mother of 41years (but better odds that the 1:4 quoted).

Now what we haven't considered is how likely the grandparents are to actually be carrying in a disease causing recessive gene in the first instance....probably significantly less that 1:2 (which our model assumes) - as genes which cause severe abnormalities are present at low frequency in the population (because traditional outbreeding evolution gets rid of them). So providing the frequency of a bad recessive gene in the general population is less than 1:7 the odds of a 41 year old mother having a downs syndrom baby are in fact greater than cousins having a baby with an inherited disease.

SMS- granted, but I was talking about cases where both the parents do carry the same recessive disorder, and the fact that if one did, one might very well want to know about it. The chances of having a child with such a disorder, in the absence of any evidence of such an abnormality in either family, are still small even for cousins marrying, but very significant if those cousins happen each to carry the same recessive disorder. Hence my remark about genetic counselling and potentially the decision not to marry a cousin/ another carrier.

And if you knew that several babies and children in your family had been sick or died for no known reason, you might seek screening before even marrying if you knew that it might be inherited, and that there might be a higher chance of your children being affected if you marry person X rather than person Y.

Having said this, the most common recessive disorder in this country is still Cystic Fibrosis, which is carried by a significant number of people unrelated to each other. By rights, we should be screened and counselled before having any children.

Onestone, I apologise for the terse tone of my earlier posts - am in a grumpy frame of mind today. I absolutely support anyone wanting to learn and understand more about anything, particularly science, as I think scientific literacy is much too low amongst the general population.

And I think this topic is very interesting, and worthy of discussion. But it doesn't mean I agree with the way in which you've taken your case in what appeared to me to be quite an entrenched fashion - my disagreement with that is part of the debate, though I could have worded it better.

As for screening; I've been in the horrible position of discovering at a 20 week scan that my daughter had a fatal neural tube defect. Until that point I had no idea just how terrible it would be to agree to a termination, which broke my heart, even though I knew that nothing would change the fact that she would die, if not before birth, then very shortly afterwards. I honestly could not terminate for anything less than a lethal condition, having had that experience, and so I guess simply talking of screening fails to take into account the complex emotional and social issues involved.

On the issue of cousins marrying - in cultures where the pattern of consanguinuity in marriages is common, it may well be argued that more screening and education would be valuable; banning for the purpose of preventing genetic defects does, I think, throw you open to the charge of not treating at-risk groups equally, especially where the risk of some conditions is far higher than the risk to the children of two related individuals.

Public policy does, I think, have to at least attempt to be fair and even-handed, and your proposed policy to me does not meet those criteria.

I think that this has highlighted one important thing. Many people do understand the impact and probalities of the short term impact if the cousins are known carriers - but have less understanding/perspective (for wnt of a better word) when it comes to understanding that teh cousins probably aren't carriers.

FWIW I have enjoyed the debate....and I do think that many people - including OneStone have learnt something and I hope perhaps modified their views accordingly, with a little more perspective.

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Sound like some pretty good conclusions to me. Interesting thread, despite my snippiness!

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I thought it had all kicked off again there.

I think that is a worthy summing up OneStone

I believe that Catholic canon law forbids cousins marrying (though, of course, they may still do so in civil services).

Freezing eggs is a very expensive and rather unpleasant option - speaking as one who has an IVF baby (same egg retrieval process and then you would need IVF to conceive) - hmmm - I was over 40, IVF (which may have some increased risks associated) and heavy genetic load of mental issues in the family - probably should have been sterilised at birth!

Have been reading this thread with some interest and some shock at some people's views. I have nothing new to add but feel that I should stand up next to DanJARMouse and state that I too am happily married to my first cousin!

We were brought up at opposite ends of the country and got to know each other when I was 18 and he was 25. Initially he was just being friendly, introducing me to his friends in the pub and taking me along to parties during a Christmas holiday when our parent's decided to meet up after many years. It became obvious to us that we enjoyed each other's company, so we kept in touch, fell in love and we've been married for 5 years (together for 10) and are expecting our second baby - our first has no genetic problems.

There is no history of any genetic problems in our family and so by combining our (admittedly shared!) gene pool we didn't feel that we're running any exceptional risk. Perhaps if there was we would have considered some degree of screening or counselling.

As with DanJarMouse, I think of my in-laws as my in-laws and don't give it much thought at all! The situation is occassionally brought up by friends in the pub as an excuse for a bit of gentle teasing but this has become less and less as the years have gone on.

It makes for a shorter Christmas card list too!!

my sister in law has been married to her first cousin for nearly 30 years and have 3 well adjusted bright kids they put up with predjudice from other famiy at the start but came out proving it can work

I couldn't help noticing this line.

"Several posters have reported how, as a matter of fact, in their life they encounter children with severe illness due to inbreeding"

Wrong!. 'Some posters reported that they knew of children with severe illness whose parents were slightly related'
They also knew of children with severe illness whose parents were NOT related and lots of children of slightly related parents who were not ill at all.

There was no evidence that the two things were connected at all.

Logically there will be a marginally higher risk for people who are related, but for that matter there is a higher risk if you marry someone from your village, your town or your country as they will be closer genetically than a stranger picked by lottery from the other side of the planet.

In any case I believe the true objection when this comes up is and only is the 'yuk factor'. This is at root a religous objection and therefore has nothing to do with logic or facts. People just look for a genetic excuse to support what their prejudice has already decided.

Ask someone who objects how they will feel if one of the couple is sterile and they will usually still object because it makes them uneasy.

OK - it doesn't account for spontaneous mutation. And when I mean stabilise - I kind of mean in a genetic sense. Genes that are really bad can disappear if it produces individuals that can't or infrequently reproduce.

The benefits of that stable population depend on the genes that are in the gene pool. If there are no nasties then that genetic stable population will be a healthy one. If their are lots of nasties then that "stable" population probably won't be competitive in teh environment it lives in.

It does get complicated. Of course spontaneous mutations will start to contribute to teh gene pool - that would be the only way to evolve. But if there is inbreeding those mutations will become part of that stable population. I was thinking about stabilising the starting stock.

2 examples where inbreeding has been used as a mechanism to obtain a stablilise population - although artificial - are in selecting robust crop strains and selecting laboratory strains of mice and rats (although mice and rats do tend to be considered quite weak if they were to be put up against a naturally occuring strain I think - again not my field - the inbreeding is done for the scientists benefit here - so TBH not exactly what we are talking about here).

It is true that successive inbreeding can be good and bad - depending on teh circumstances. What I was trying to illustrate was that their are examples where inbreeding has benefits.