Q. Why would a mutation in the BRCA1 gene that is associated with breast cancer be found in Ashkenazi Jewish women at a much higher rate than in other Jews?

A. The specific mutation of the gene apparently arose in Central and Eastern European Jews around the time that just about 350 of the ancestors of modern Ashkenazi Jews were alive, recent genetic studies suggest. That event, combined with the longstanding tradition of marrying within the group, could have facilitated a wide perpetuation of the harmful gene among the few founders’ many descendants.

A 2011 study of modern carriers’ DNA, published in The European Journal of Human Genetics, concluded that the mutation, which can be traced to one prehistoric founder, was apparently present in other European populations for hundreds of years but entered the Ashkenazi gene pool in Poland 400 to 500 years ago.

At the late medieval period, according to a 2014 study published in Nature Communications, there was what the researchers call a bottleneck in the Ashkenazi population of around 350 people, followed by a rapid expansion. The study looked at the entire genomes of 128 modern individuals and compared long shared segments of DNA, which are common in Ashkenazi people, to reach their conclusions.