Jan 22, 2018 | Updated: 09:54 AM EDT

All In The Family: Arranged Marriages And Genetic Diversity

May 14, 2015 01:40 PM EDT

A Massey University research team has discovered some interesting new truths about the ways arranged marriages affect genetic diversity and the ways that humans follow even important cultural rules selectively-and they may surprise you. The results show that the isolated Indonesian Rindi tribe produces genetic diversity similar to random mating by loosely complying with their rules which mandate arranged, inbred marriages.

Although in modern developed nations inbreeding is uncommon, in societies which favor and arrange marriages between members of extended families, inbreeding is more common. It has long been accepted that inbreeding, the mating of people who are closely related to each other, can lead to damaging and even dangerous genetic outcomes. However, until this new study no one could say for sure how much impact arranged marriages might have on genetic diversity; most operated on the simple assertion that the results would be bad.

For example, people often cite the higher rates of color blindness and hemophilia among royal families in Russia and the UK as evidence of the often damaging effects of inbreeding on genetic diversity.

The Massey team studied the traditional Indonesian Rindi culture, which exists, mostly isolated, on the island of Sumba. The Rindi only number in the thousands, making the genetic issues potentially more acute. Rindi rules for marriage state that men should marry their maternal first cousins; the goal of this rule is to bring the bride's and groom's families together in the same, more power sphere of influence by pooling power and wealth.

The researchers used SMARTPOP, a computer simulation tool, and DNA sequencing to answer two central questions. First, what genetic consequences might arise from following Rindi marriage rules? And second, how closely do the Rindi adhere to these rules? The team predicted that the marriage rules would reduce genetic diversity.

However, this intuitive and educated guess turned out to be wrong. The Massey team data proves that there is a fairly relaxed attitude to these marriage rules, and that about two in three follow them. And even with this significant adherence to the rules, this compliance is sufficiently relaxed so that it produces genetic diversity which closely resembles random mating.

"People like to say they follow the rules, but actually we're all really good at looking the other way if people don't. Our work suggests that sometimes that's a good thing," Cox says.

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