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Cousin marriages in the UK: what are the genetic risks?

22 June 2008
By Dr Alison Shaw
Department of Public Health, University of Oxford.
Appeared in BioNews 463
Marriage between relatives such as first cousins increases the risk in children not of general birth defects and genetic problems of all kinds but of what geneticists call 'recessive' conditions: those caused by inheriting two copies of a gene each of which carries a mutation. It seems we may each carry about 12 mutations associated with recessive conditions. There are about 3.5 thousand recessive conditions, most of them very rare, so the chance of both partners carrying a mutation in the same gene is low. If both parents carry the same recessive mutation they then have a one in four risk of an affected child each time they conceive a child. In the UK, most children with recessive conditions are born to unrelated parents.

In children of unrelated couples, the risk of congenital and genetic including recessive problems is usually estimated at about 2-3 per cent. First cousins have a higher risk of recessive problems in children than unrelated couples because they share 12.5 per cent of their genetic material and may have inherited the same mutation from a common ancestor. Risk to their children is usually estimated at about double the risk of congenital and genetic problems in children of unrelated couples. At about 4-6 per cent, this doubled risk is also a 94-96 per cent chance that their child will be perfectly healthy. A cousin couple is thus more likely to have a healthy child than to have a child with a recessive problem.

Risk for recessive conditions also varies between communities depending on the distribution of recessive mutations, making some conditions more common than others, and on the history of marriages to blood relations within them. In the UK, cousin marriage is no longer common in the majority population but is practised within quite diverse minority groups, particularly those of relatively recent migrants from the Middle East and South Asia. Debate has focused on British Pakistanis, the largest population practising cousin marriage, because of the comparatively high incidence of a range of serious recessive problems.

In some British Pakistani families, marriage within the family or extended kinship group has been practised for generations and first cousin couples can trace numerous additional blood ties in their family history. They may therefore share more than 12.5 per cent of their genetic material and have a higher risk for recessive problems in their children than if they were first cousins without any additional connections by blood. This is the most likely explanation for the observation that, in some sections of the British Pakistani population, the risk of death or serious disability in children may be as high as 10 per cent.

Marrying an unrelated person does not remove this risk because of the general population risk of two people carrying a mutation in the same gene, particularly for the more common recessive conditions, but it can significantly reduce risk for rare recessives. In managing recessive risk for individuals or couples, the critical thing is not so much whether or not they are blood relatives but whether or not they are carriers of a mutation for the same condition. Premarital or pre-conception genetic counselling can be useful to establish a couple's risk and reproductive options before they start a family, particularly where one or both partners have an affected relative or if one partner has a recessive conditions themselves.

In the UK today, it would be as inappropriate to castigate cousin marriages on grounds of elevated recessive risk as it would be to stigmatise late childbearing on grounds of elevated risk of chromosomal abnormalities. Down syndrome risk is now managed through generic prenatal screening and counselling. Recessive risk presents rather different challenges because it includes risk for many different, mostly rare, conditions, some of them treatable. Carrier and prenatal tests are now possible for an increasing number of recessive conditions but screening for them all it not currently possible or realistic. It is often only after the birth of an affected child that clinicians know which condition is relevant within any particular family.

Recessive risk need to be managed by improving awareness of it, including the elevated risk associated with marrying relatives such as first cousins where there is a history of marriage within the family, and by ensuring appropriate genetic counselling. Public health and genetic counselling provisions should strive to enable individuals and couples to make informed marital and reproductive decisions, wherever possible. Any couple worried about genetic risk to children should ask their GP for a referral to a genetics clinic. Some scientists think that in the next decade or so we may all have access to our own personal DNA results and will each consequently know much more about the mutations we all carry.

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Oxford Desk Reference: Clinical Genetics. Oxford University Press |  27 January 2022
2) Bennett, R.L. et al. (2002) Genetic counseling and screening of consanguineous couples.
Journal of Genetic Counseling, 11 (2): 97-119 |  27 January 2022
3) Devereux, G. et al. (2004) Variations in neurodegenerative disease across the UK: findings from the national study of Progressive Intellectual and Neurological Deterioration (PIND)
Archives of Diseases in Childhood. 89: 8-12 |  27 January 2022
4) Bundey, S. and H. Alam (1993) A five-year prospective study of the health of children in different ethnic groups, with particular reference to the effect of inbreeding.
European Journal of Human Genetics, 1: 206-19 |  27 January 2022
6 September 2010 - by Anshu Rastogi 
'When Cousins Marry' (Dispatches) was a difficult film to make. It was painful at times but immensely rewarding in the end. The film was commissioned because it highlighted a potentially avoidable cause of serious disabilities in hundreds of British children each year...
31 August 2010 - by Professor Sandy Raeburn 
Why am I criticising a campaign to reduce the incidence of severe autosomal recessive diseases? After all, I spent five years of my 40-year clinical career in medical genetics living in Oman - a Muslim country where over 50 per cent of marriages are consanguineous? Let's dig deeper!...
10 May 2010 - by Victoria Kay 
Charles Darwin's concerns that his children's ill health was due to his cousin marriage were justified, according to a new study. The UK-Spanish study, which analysed four generations of Darwin's family, provides statistical evidence of a link between ill health and the degree of inbreeding in his and his wife's families....
22 March 2010 - by Professor Marcus Pembrey 
A report and analysis promoting Baroness Ruth Deech's views on cousin marriage, published in the Times newspaper on Saturday, makes some serious errors and does nothing to either clarify the true health impact of cousin marriage or help couples at risk of recessive genetic conditions...
22 March 2010 - by Dr Vivienne Raper 
A leading bioethics professor and crossbench peer is to reignite the debate on the genetic risks of marriage between first cousins...
9 June 2008 - by Ailsa Stevens 
Over a billion people worldwide live in regions where 20-50 per cent of marriages are consanguineous, and first-cousin unions are especially popular. Discussion of this phenomenon is confused by the fact that its causes are social and economic, while its outcomes tend to be measured in terms of child...
12 December 2005 - by Alastair Kent and Dr Pritti Mehta 
British Pakistanis are under the spotlight yet again, this time not for alleged links with terrorism, but for the practice of cousin marriage. Last week in BioNews, Ann Cryer, Labour MP for Bradford, re-presented her case against cousin marriage. This followed the 'Newsnight' special report (16 November) highlighting that British...
5 December 2005 - by Dr Aamra Darr 
The recent Newsnight programme (broadcast on BBC2, on 16 November) on cousin marriage attempted to deal with a complex health issue, involving the marriage preference of a minority ethnic group, genetic risk, lay and professional understanding of this risk and the attempts to deal with it. Genetics is a relatively...
1 December 2005 - by Ann Cryer MP 
In 2002, 42.2 per cent of all births in Bradford were to families of Pakistani origin, and a further 5.8 per cent were to others from the Indian subcontinent. The incidence of deafness amongst Asian children in Bradford is 4.60/1000 (compared with 1.38/1000 amongst non-Asian children). For cerebral...
18 November 2005 - by BioNews 
A British politician has said that marriages between first cousins should be outlawed because of the increased risk of genetic disorders in their children. Ann Cryer, the MP for Keighley, Bradford, told the BBC's Newsnight programme that British Asians should be persuaded to abandon the tradition. A report commissioned by...
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