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Health

Ways to skin the cousin marriage issue

Gene ExpressionBy Razib KhanMay 13, 2008 4:33 AM

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Another article about cousin marriage in the UK. The issue here is simple; you have a National Health Service which covers everyone, and doctors are noticing that Pakistanis are overrepresented in many cases of recessive diseases. The culprit is probably cousin marriage. Here are two points which are both valid:

'In our local school for deaf children, half the pupils are of Asian origin though Asians only form about 20 per cent of the population,' said Ann Cryer, MP for Keighley. 'I also know of several sets of parents in my constituency who are cousins and whose children are severely disabled. I have no doubt that the mothers and fathers being closely related to each is a key factor. ... This last claim is hotly disputed by genetic counsellors and Muslim doctors. They point out that the danger of a child having birth defects if the parents are cousins is double that of other children, which means the risk rises from about 2 per cent in the general population to about 4 per cent when the parents are closely related. A risk of 4 per cent therefore does not make it 'likely' there will a genetic problem, as Woolas claimed, say genetic counsellors.

What's going on here? First, it is correct that though first cousin marriages increase the likelihood of deleterious diseases in the offspring multiplicatively, the chances are still relatively low. On the other hand, in terms of absolute numbers the number of those with a given recessive disease may increase multiplicatively as well. In other words, though on the individual scale the expected risk is low, when it comes to population level outcomes ubiquitous cousin marriage is going to mean a society where a range of recessive diseases are far more common. You can predict the proportion of individuals with a recessive disease who are the products of first cousin marriage by the following formula: K = c(1 + 15q)/(c + 16q - cq) The proportion of first cousin matings being c within the population, and q being the frequency of the recessive allele in question. Here's some numbers for the United States.... Condition - % of affected children whose parents were first cousins Total color blindness - 15 Albinism - 21 Xeroderma Pigmentosum - 23 Ichthyosis Congenita - 35 Tay Sachs - 40 There is one major caveat here: these assertions apply to first cousin marriages where the individuals are related through one line of descent. That is, in the United States when individuals marry their cousins, they are usually marrying someone with whom they share one set of grandparents. And that's it. In many societies you have large inbred clans where people are related along many lines of descent, and this tends to amplify the characteristics of inbreeding. I think this is the real long-term problem when you have culturally sanctioned cousin marriage; it will occur generation after generation so that pedigree collapse may be inevitable. In any case, this portion of the article made me wonder:

'The danger posed by cousin marriage is highly exaggerated,' said Aamra Darr, a senior research fellow at Bradford University. As Darr pointed out, women in Britain are more likely now to have children when they are over 30, increasing the likelihood of them having babies with Down's Syndrome. But no one suggests that there should be a ban on over-30s having babies, Darr added. Medicine has adapted to improve screening services for these women. 'We should recognise that for British Pakistanis, cousin marriages represent significant cultural advantages. Recent advances mean we can pinpoint many of those at risk of having affected babies. That is where we should be placing our efforts.'

What exactly should individuals do when they find out that their future child is going to carry a recessive condition? Is this doctor suggesting that selective abortions should be the appropriate response to adapt to the ubiquity of first cousin marriages in the Pakistani community? Or pre-implantation screening? Related: Other posts on cousin marriage.

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