By Emma Wilkinson
BBC News health reporter
Some want better genetic screening for children of first-cousin marriages
The contentious issue of first-cousin marriage is set to hit the headlines with a heated debate over their impact on genetic disorders in Britain.
Some have called for such "consanguineous" marriages to be banned but others argue better genetic screening is what's needed.
Later this month, the Royal Society of Medicine and Progress Educational Trust are holding meetings to examine the evidence.
Over a billion people worldwide live in regions where 20%-50% of marriages are consanguineous - that is where the partners are descended from the same ancestor.
In the UK, first-cousin marriages are common in some populations such as Pakistani communities.
The tradition is also common among some other South Asian communities and in some Middle Eastern countries.
It is known that interbreeding raises the risk of genetic illness.
But some experts say this risk has been wildly exaggerated.
Professor Alan Bittles, director for the centre for human genetics in Perth, Australia has collated data on infant mortality in children born within first-cousin marriages from around the world and found that the extra increased risk of death is 1.2%.
In terms of birth defects, he says, the risks rise from about 2% in the general population to 4% when the parents are closely related.
"There has been a bit of a push of people saying that consanguineous marriage is dangerous and should be banned but before we start making decisions on that we need to look at the data," he says.
The genetic conditions more common in populations with high rates of consanguineous marriage are rare recessive disorders which cause a wide range of problems, such as blindness, deafness, skin diseases, and neurodegenerative conditions.
We all carry gene mutations and sometimes, such as in the case of cystic fibrosis, they are fairly common in the general population.
But when a population has a small gene pool the gene mutation can become more frequent.
If two people carrying a recessive gene reproduce, their children have a one in four chance of having the disorder and a one in two chance of becoming carriers themselves.
Professor Bittles explains that within some communities even non-cousin marriages carry higher risks of these rare genetic disorders because those involved are from the same "biraderi" or clan.
He supports better genetic screening services for those affected over any kind of ban or limit on first-cousin marriages.
"It would be a mistake to ban it, people have been following consanguineous marriage practices for hundreds of generations and then we come along and say you are going to have to stop - no one will pay attention.
"I would go for selective screening it would be much more effective."
One of the problems in the debate is that many figures get bandied about but really good data on the health effects of first-cousin marriage is lacking.
The Born in Bradford study, tracking 10,000 children from birth through their first two decades of life aims to rectify this.
Around 50% of children born in Bradford are to Pakistani parents.
Dr Peter Corry, a consultant paediatrician at Bradford Teaching Hospitals says they have identified almost 150 of these rare genetic conditions in the city - much higher than would be expected.
And data collected by the British Paediatric Surveillance Unit has shown since 1997 there have been 902 British children born with neurodegenerative conditions and 8% of those were in Bradford which only has 1% of the population.
He too advocates better genetic testing for families affected.
"Different populations have different patterns of genetic conditions.
"In Bradford, these conditions are much more prevalent than you would expect but they're still pretty rare.
"I would guess there is a genetic or metabolic test for about half the conditions and each time in Bradford a new gene is discovered there are two or three families who can be offered testing," he added.
The Born in Bradford study is collecting information on the family tree of those taking part but will also look at environmental issues which may have an impact on health.
"Of the first 1,100 pregnant Pakistani women recruited, 70% are from consanguineous marriages," he said.
"When I came to Bradford 20 years ago most Pakistani parents didn't understand when you were talking about genetics but now we're seeing young Pakistani professionals with a much greater knowledge about genetic conditions."
Dr Corry says the numbers may be small but the genetic disorders can be devastating for the families involved.
In one extended family he works with six children who have the same genetic condition which means they are unlikely to live beyond their teens.
But he says on the positive side, affected families who work with the genetic services in Bradford are helping to advance doctors knowledge of such conditions which leads to new genetic tests and potential treatments.