12:24pm Wednesday 23 October 2019

Cousin marriage puts gene pool at risk

Although frowned on by many, including some religions, the only country in the western world to actually outlaw first cousin marriage is the United States.

Author and poet Edgar Alan Poe, physicist Albert Einstein and, perhaps ironically, the author of the theory of evolution, naturalist Charles Darwin are among famous people in history to marry their cousins.

New research published in the journal Molecular Biology and Evolution examined the relatively small population of the Rindi region on the Indonesian island of Sumba, where marriage rules dating back centuries dictate that men must marry a first cousin on the mother’s side of the family.

With a population of only about 7000, the scientific team, which includes experts from Indonesia, Singapore, the United States and New Zealand, expected to find significant biological effects of hundreds of generations following the rule.

They used a specially developed computer program to run simulations to see how non-sex linked chromosomes (autosomes), the X and Y chromosomes and the DNA that is inherited only from the mother (mitochondrial DNA) would be affected if everyone followed the rule. They modelled it over many generations because the evidence suggests the practice is extremely longstanding.

What they found was that arranged marriage should theoretically reduce the genetic diversity in those different DNA regions but what surprised them was that testing of the DNA in the current population did not reflect that.

It turned out the saving grace of the population is that the marriage rules are not strictly followed, either because it was not possible if a man’s mother had no siblings or no siblings with daughters, or because some men chose to ignore the rule – perhaps to forge alliances between families in other ways.

Senior author on the paper Associate Professor Murray Cox from Massey University’s Institute of Fundamental Sciences says anthropologists have been studying the diverse range of marriage rules for the last century but this is the first study to try and understand any biological effects.

“Marriage rules help structure connections within and between communities but any rules that affect marriage will also have a direct impact on offspring and the genetics of a community,” Dr Cox says.

“It’s important to understand these genetic effects because reduced diversity might result in lower fertility, genetic disorders and potentially the loss of a community.”

A 2013 paper published in The Lancet found that babies of first cousin marriages were twice as likely to have a genetic disorder such as cleft palates, genital defects or problems with their nervous, respiratory and digestive systems. However, this percentage was fairly low at 6 per cent and is comparable to the risk faced by children born to mothers over age 35.

The cultural practice of arranged marriage is common in areas like South Asia or the Middle East as well as some Western countries including New Zealand, the United Kingdom and the Netherlands where there are large populations from these regions.

The research was lead by scientists from Massey University’s Institute of Fundamental Sciences alongside scientists from the Eijkman Institute for Molecular Biology (Indonesia’s top genetic research centre), the Division of Biotechnology at the University of Arizona, United States, and the Complexity Institute at Nanyang Technological University, Singapore.

Dr Cox has also produced a summary of the teams’ research in the form of a video.

Massey University.

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