A single man or woman, living in Scotland three centuries ago, produced the genetic defect that places some families at far higher risk of cancer.

While the so-called "Celtic gene", traced by researchers at St Andrew's University, is linked to only a tiny number of breast cancer cases, it is hoped that finding the common ancestor will help doctors screen families for the condition.

It is well known that defects in two tiny parts of the human genetic structure, labelled BRCA1 and BRCA2, are linked with increased risk of breast cancers.

Overall, between 3% and 4% of breast cancer patients have one or other of the damaged genes.

Tiny numbers

The "Celtic gene" mutation accounts for approximately one in ten of those with a damaged BRCA1 gene.

The St Andrew's team have found 20 families with a history of breast cancer, all of whom have this particular genetic mutation.

Although some of these families had at some point moved abroad to Canada, checking back through their family trees revealed that they all came from the south-west of Scotland.

Other genetic similarities suggested they all had a single, common ancestor about 300 years ago.

Unfortunately, as genealogical records in Scotland are poor before 1800AD, there is little hope of finding the precise source of the mutation.

However, the practical benefit, according to Professor of Medical Science Michael Steel, may be to help spot the same or similar genetic defects in other families without the need for a battery of expensive and time-consuming tests.

A family which originated in the south-west of Scotland, for example, might be offered this a test for this particular gene defect.

"We could devise a screening system which starts with the mutations which they are most likely to have," said Professor Steel.

The Scottish population appears to be a good place to trace these mutations, as, compared to England, for example, there has not been widespread intermarriage between people in different parts of the country, or a great influx of genetic material from other countries.

"It confirms that it is worth looking for these mutations in Scotland," said Professor Steel.