A remote valley in southern Colorado may not be the first place one would go in search of a lost Jewish community. But a recent study published in the US Journal of Human Genetics suggests that San Luis is harbouring exactly such a secret.

"We found evidence that DNA segments are shared by Sephardic Jews and Spanish Americans from Colorado and New Mexico, suggesting shared ancestry," said Dr Harry Ostrer, Professor at the Albert Einstein College of Medicine in New York and the director of the study.

Dr Ostrer's team analysed two communities whose ancestry can be traced back to Spanish colonial times, one in the San Luis Valley, which stretches between southern Colorado and New Mexico, and one in the Loja Province of southern Ecuador.

It is the first time that researchers have studied the entire genome for large chunks of DNA that indicate shared ancestry, rather than just looking at particular disease mutations or other individual genes. They calculated Jewish ancestry among the Lojanos at 5-10 per cent, and among the Spanish Americans, or Hispanics, at 1-5 per cent.

Rumours of a secret Jewish past had been flying around the San Luis Valley for decades. Five hundred years ago, a number of Conversos - Jews who were forced to convert by the Spanish Inquisition - made their way across the Atlantic. Over the past few decades, scholars claim to have found remnants of crypto-Jewish practices in communities in south-west USA, as well as in Latin America. As the theory goes, when the Inquisition hit the shores of the New World, the crypto-Jews moved to the far flung corners of the Spanish Empire in a bid to escape the church.

Some Hispanics are even said to keep up forms of Jewish life without knowing their origin. Demetrio Valdez, a Catholic cattle farmer in the San Luis Valley, for example, grew up practising kosher slaughter without realising that the technique was Jewish.

Dr Ostrer's study was undertaken after previous studies found that a mutation for breast cancer which is usually associated with Ashkenazi Jews - 185delAG - was prevalent in New Mexican Hispanics too.

Scientists estimate that the mutation arose well over 2000 years ago among the Hebrew tribes of Israel prior to any split between Ashkenazim and Sephardim. In his new book The Wandering Gene and the Indian Princess: Race, Religion and DNA, Jeff Wheelwright explores how the mutation may have travelled from the Middle East.

"I found that people are not always who they say they are when we apply genetic criteria to them. But in the end, the identity that people claim for themselves is indeed who they are. If genetics helps someone to make or change an identity, so much the easier," said Mr Wheelwright.

Orlando Mondragon, one of the New Mexicans who took part in the study, wasn't at all surprised to hear news of his likely Jewish heritage. "He always thought so," said his wife, Viola.