Sitting In his office with a Jewish couple last year, geneticist Professor Ohad Birk asked if they would like to speak to an Arab couple.

There was a particular reason for his inquiry. He had discovered that the two couples shared ancestors dating back almost a millennium. Prof Birk made the staggering discovery after each had come to him as parents with two seriously ill children suffering from strikingly similar "totally new diseases".

He researched their disorders and identified the mutation. The two cases were identical. "Of the three billion letters of the human genome, half a million letters surrounding this mutation were totally identical between these two families," he says.

So how did a Bedouin Arab family and an Ethiopian Jewish family end up with the same mutation? "We were able to show that these families were related from hundreds of years ago," he explains. It made historical sense as there were trade ties between the the Ethiopian empire and Egypt. The likelihood is that a man and a woman from these countries had at least one child together and were shared ancestors for the Jewish and Arab families.

One couple lost their two children to the disorder. The other couple's children are now battling serious illness. But thanks to Prof Birk's research and genetic counselling, the Jewish family is also now raising a healthy child and the Bedouin woman is pregnant with a healthy baby. The phone call between them was emotional and awkward. "What do you say to a relative from all those years ago?"

The research is changing assumptions

Prof Birk is the genetic detective of the Negev, a region inhabited by Jews from all over the world. As head of both the Genetics Institute in Soroka Medical Centre and the National Institute for Biotechnology in the Negev at Ben-Gurion University, he has identified 30 new diseases and more than 100 mutations. Among them is a group of mutations that parallel Tay-Sachs in their carrier rate.

In the Moroccan and Iraqi Jewish communities there are babies born healthy, with brains that seem normal from MRI scans. The brains then begin to atrophy and most die between the ages of 10 and 20. Until Prof Birk solved the puzzle six years ago, geneticists were stumped and families were often unaware that their child's condition was genetic. His team realised they were dealing with two different mutations in the SEPSECS gene. These were causing the disease Progressive cerebellocerebral atrophy (PCCA). Around one-in-40 Jews of Moroccan and Iraqi descent are carriers.

In 2011, months after Prof Birk's discovery, Israel started offering free genetic testing for PCCA. But there were young Jews from Moroccan families with similar symptoms whose parents did not carry PCCA. His team discovered another disease was the culprit.

In 2014, Miora Feinstein, a doctoral student under his supervision, identified two recessive mutations in the VPS53 gene that are responsible for PCCA Type 2, and found that one-in-37 Jews of Moroccan origin carried both mutations. State funded screening was again quickly made available. As with the case of the long-lost Arab and Jewish relations, this discovery took Prof Birk back into history.

One of the two mutations was traced to Spain and he believes that carriers are descendents of families who left in the 1492 Spanish Expulsion. This history is not just a curiosity; it also provides potentially life-saving leads.

He is now assessing the frequency of PCCA Type 2 in other Jewish communities with origins in Spain, including Bulgarians and Turks, and how they may be helped through screening.

Prof Birk also works on mutations carried by Arabs far beyond Israel's borders. He has also taken part in the BBC's Doha Debates programme which is seen across the Arab world, discussing marriage between cousins.

Not all of Prof Birk's findings relate to life-threatening conditions. He has helped Jews of Indian origin understand why some have sight problems that cannot be rectified with glasses, after an Indian-Jewish family showed up with a sight problem. As Dr Birk and his colleagues were mulling over the case, the ophthalmologist from his team happened to go shopping. At the counter was a "nice Indian girl whose eyes were like the family's". Her eyes, like theirs, moved very quickly.

Suspecting that this could be a disorder affecting many families, the researchers set out to find the mutation in a larger population. They found that one-in-10 Jews from Mumbai families carry a recessive gene that when passed on to offspring from both parents causes a disorder affecting both short- and long-range vision.

His research is changing assumptions. In 2009, Haaretz quoted a doctor and historian of Sephardic Jewry saying "there is no disease you can call a Sephardic genetic disease". It would be difficult to argue that today.

"For me, it's amazing that you could still discover the most common diseases in the last five years."