Cousin marriage: What new evidence tells us about children's ill health (BBC)

 

In a busy, terraced house in Bradford, three sisters are animatedly chatting. It's a big day at their home: a beautician sits on their sofa, styling their hair and makeup. The room is warm with fun and laughter. It feels like a scene from a Jane Austen novel: three women in their late 20s, each of them bursting with personality, swapping stories.

And like most Austen novels, the conversation often turns to marriage.

The sisters are preparing for a family wedding at the weekend - where the bride and groom are first cousins. Many people might find this unusual, but in their family and in some parts of Bradford, it's fairly common.

Ayesha, who at 29 is the oldest of the three sisters, also married her first cousin in 2017. She has two children with her husband and their marriage is happy, she says. It felt perfectly normal at the time to marry her cousin. Their mother, a Pakistani migrant, assumed it was what all three of her daughters would do.

But 26-year-old Salina, the youngest of the three, tells us she broke the mould by having what they call a "love" marriage, choosing a partner from outside the family. Salina tells us she is outgoing and ambitious; marrying a cousin simply did not appeal to her. Then there's Mallika, who at 27 is the middle of the three. She's still single and has already decided not to marry within her family.

"I said to my mum that I wouldn't judge my sisters but I wasn't going to do it," Mallika tells us. She says having an education has created opportunities for her. "Before, even if you had an education, you wouldn't be expected to carry on with it. You would be thinking of marriage. Now the mindset is so different."

Worrying new data

In the UK and across Europe, cousin marriage is coming under increased scrutiny - particularly from doctors, who warn that children of first cousins are more likely to experience an array of health problems.

And there's now some new, potentially worrying data from Bradford to add into that mix.

Researchers at the city's university are entering their 18th year of the Born in Bradford study. It's one of the biggest medical trials of its kind: between 2007 and 2010, researchers recruited more than 13,000 babies in the city and then followed them closely from childhood into adolescence and now into early adulthood. More than one in six children in the study have parents who are first cousins, mostly from Bradford's Pakistani community, making it among the world's most valuable studies of the health impacts of cousin marriage.

And in data published in the last few months - and analysed in an upcoming episode of BBC Radio 4's Born in Bradford series - the researchers found that first cousin-parentage may have wider consequences than previously thought.

The most obvious way that a pair of blood-related parents might increase health risks for a child is through a recessive disorder, like cystic fibrosis or sickle cell disease. According to the classic theory of genetics laid out by the biologist Gregor Mendel, if both parents carry a recessive gene then there's a one in four chance that their child will inherit the condition. And when parents are cousins, they're more likely to both be carriers. A child of first cousins carries a 6% chance of inheriting a recessive disorder, compared to 3% for the general population.

But the Bradford study took a much broader view - and sheds fresh light. The researchers weren't just looking at whether a child had been diagnosed with a specific recessive disorder. Instead they studied dozens of data points, observing everything from the children's speech and language development to their frequency of healthcare to their performance at school. Then they used a mathematical model to try to eliminate the impacts of poverty and parental education - so they could focus squarely on the impact on "consanguinity", the scientific word for having parents who are related.

hey found that even after factors like poverty were controlled for, a child of first cousins in Bradford had an 11% probability of being diagnosed with a speech and language problem, versus 7% for children whose parents are not related.

They also found a child of first cousins has a 54% chance of reaching a "good stage of development" (a government assessment given to all five year-olds in England), versus 64% for children whose parents are not related.

We get further insight into their poorer health through the number of visits to the GP. Children of first cousins have a third more primary care appointments than children whose parents are not related - an average of four instead of three a year.

What is notable is that even once you account for the children in that group who already have a diagnosed recessive disorder, the figures suggest consanguinity may be affecting even those children who don't have a diagnosable recessive disorder.

Neil Small, emeritus professor at the University of Br

adford and the author of the study, says that even if all of the children with recessive disorders visited their GP more than average, "this does not explain the much wider distribution of excess health care usage in the consanguineous children".

The study, he says, is "exciting because it gives the opportunity for a much more accurate development of a response, targeting interventions and treatments"

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