The Viking Gene

Two founder variants account for 90% of pathogenic BRCA alleles in Orkney and Shetland

https://tinyurl.com/54pbt2jh

For most women, the chance of carrying the BRCA gene mutation is 1:400. In certain populations, notably the Ashkenazi Jews, Scandinavians from Norway, and the Shetlanders this risk increases to 1:40, making breast, ovarian and other cancers ten times more likely.

With next generation gene sequencing, it is now possible to identify variants of the BRCA genes which are responsible for this increased risk. Using genealogy, the origins of the mutations can be traced to a founder variant. 

Founder effect is increased when populations are small, with in-breeding. In Norway this followed the ravages of the bubonic plague, for the Ashkenazi the outside stimulus was the destruction of Jerusalem. Recent research has identified the founder family in the Shetlands as settlers on the island of Whalsay before 1750. Knowing the BRCA variant means that whole of population screening can be done more cheaply.