The late Gen. Francisco Franco, Spain’s long-ruling dictator, has often been quoted as saying, “Jewish blood runs through the veins of every gentile Spaniard.”

Franco, of course, was referring to the fact that in the year 700 CE, the Muslim Moors of North Africa conquered much of what today is Spain. Many Jews also came to Spain at that time. Together, they enjoyed a period of unparalleled enlightenment. This is referred to as the Golden Age of Spanish Jewry, which continued until 1492, when the Catholic monarchs Ferdinand and Isabella consolidated what is known historically as the Christian re-conquest of Spain.

At that time, anyone not professing the Catholic faith was to be expelled. It is known through the churches own records of the auto de fe (the ritual of public penance of condemned heretics and apostates) that a large number of Jews chose to outwardly profess Catholicism, but secretly practised Judaism as best as they could. This significant subculture was vilified: the word “marrano” means swine. They were persecuted, tortured and often executed in public burnings.

Well, it now seems that notwithstanding Franco’s estimation, scientific study has proven that Jewish blood doesn’t actually run through the veins of every Spaniard – it only runs through the veins of about 20 per cent of them.

A paper published this month in the American Journal of Human Genetics reports on a study conducted by geneticists from Pompeau Fabra University in Barcelona and the University of Leicester in Britain.

It focused on the unique makeup of the Y chromosome, the chromosome that passes from father to son as a virtual carbon copy of itself. The Y chromosome of a cross section of Spanish men was then compared to the Y chromosome of both Jewish and Arab men of North Africa (Morocco, Algeria, and Tunisia), places where expelled Jews and Muslims fled. The researchers found that this chromosome was similar in one-third of instances, making Jews and Muslims together the ancestors of one-third of Spaniards.

It would be of great interest if this study could be extended to other locales where secret Jews are known to have taken refuge – for example, pockets of South America, New Mexico and, of course, around the Iberian Peninsula.

Another good starting point would be the island of Malta, where there is an unusually high percentage of typically Jewish Sephardi names among the non-Jewish population.