Marriage between relatives is seen as distasteful within some cultures, but it has been a common feature in many others for thousands of years.
However, growing evidence has shown that children born to parents from the same extended family face a higher risk of developing a range of health problems.
Research from 2008 shows that marriage between cousins in the US, Europe, Russia and Australia is less than one per cent.
In countries such as Saudi Arabia, the United Arab Emirates and Qatar, more than half of the population marry a spouse who is considered a relative.
Some of these countries and a number of African and Asian countries have the world's highest rates of birth defects - up to 69 cases in every thousand people.
Some experts say the real figure is much higher. Like its Gulf neighbours, Qatar has now made pre-marital medical tests mandatory.
Khalid bin Jabor al-Thani, the chairman of Qatar's cancer society and former deputy director of its national health authority, told Al Jazeera that inter-family marriages are tolerated because they are the product of "tribal traditions".
"The tolerance comes from people who used to live in very remote areas and tribes would always want to keep their blood within the family and not go outside," he said.
"In Islamic religion it is always advisable to go outside the family. But since this has [been happening] for such a long time ago, and has been carried forward, it [is] one of the issues that people overlook."
Al Jazeera's Charles Stratford reports from Doha.