Due to the "founder effect," the genetic makeup of Ashkenazi Jews is comparatively homogenous, making it easier to identify the location of genetic variations. For example, the gene mutations for early-onset ovarian cancer and breast cancer were identified via research done among Ashkenazi women.
The "founder effect" occurs when a population has a limited number of forebears and/or when one or more events substantially reduces a population. A group of people whose ancestry includes either or both of these characteristics is comparatively homogeneous in its genetic makeup. In founder populations, a larger proportion of individuals will exhibit similar, distinctive genetic traits. Also, a distinctive genetic trait is more likely to be the result of the same genetic variation within the founder population.
Ashkenazi Jews come from a small number of founders and during much of their history experienced significant population reductions. Although shrouded in history, the number of founders of the Ashkenazi Jewish population is believed to be extremely small. One study indicates that about 40% of today's Ashkenazi population come from just four "founding mothers."
By the 16th century, almost all Ashkenazi Jews were located in the Pale of Settlement. The Ashkenazim had encountered a combination of plagues, wars and anti-Semitic persecution, contracting the population size to an estimated few hundred thousand. In the ensuing 500 years, however, these numbers increased dramatically, and today stands at approximately 8,000,000.