Over a billion people worldwide live in regions where 20%-50% of marriages involve partners descended from the same ancestor

In Britain, 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. 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.