About two-thirds of Jews today – or about 10 million people – are Ashkenazi, referring to a recent origin from Eastern and Central Europe. They reside mostly in the United States and Israel. Ashkenazi Jews carry a particularly high burden of disease-causing genetic mutations, such as those in the BRCA1 gene associated with an increased risk of breast and ovarian cancer.
This genetic burden suggests that the population was shaped by what geneticists call a founder event or a bottleneck. In other words, a small number of foremothers and forefathers contributed much of the modern gene pool. As the population grew and the descendants of these founders had many children, disease mutations that were carried by the few founders became widespread.
