A law aiming to ban first cousin marriage in the UK is set to be introduced to the House of Commons on the basis that it leads to higher rates of birth defects.

Tory MP Richard Holden will introduce the Marriage Prohibited Degrees of Relationship Bill to the lower house of parliament tomorrow.

Trinity College associate professor of genetics Kevin Mitchell told Moncrieff that cousins are more likely to share genetic mutations and pass them down to their children.

“There’s all kinds of mutations floating around in the population, just defects that have occurred at some time in the past,” he said.

“Most of the time, if people are not related, when they marry each other, then these mutations don’t come together.

“When people are cousins, of course, they’re much more likely to share those - and then the offspring are more likely to inherit two copies of a mutation that can cause a disease.

"Whereas if you just inherit one copy, it’ll be fine.”

Messenger RNA or mRNA strand 3D rendering illustration with copy space.

Prof Mitchell explained that this can be especially problematic when first cousin marriage is repeated.

“The problem in some communities is that first cousin marriages doesn’t just happen once – it's a continuous sort of thing, and what that does is that increases the risk overtime,” he said.

“It kind of concentrates whatever genetic mutations there may be into quite a small pool that’s heavily shared between all the people within that population.

“Then the rates can get a lot higher – like, seven to eightfold higher.”

Genetic variation

According to Prof Mitchell, rates of genetic conditions can be higher in Europe due to less historical genetic variation.

“So, in Africa for example, there’s loads and loads of genetic variation,” he said.

“Whereas when ancient humans migrated out of Africa, that created a population bottleneck - and so in Europe, there’s much less genetic variation.

“Which means that rates of these conditions can be higher.”

Prof Mitchell said first cousin marriage is not outlawed in Ireland or most of Europe, probably because most people consider it taboo.