Because of reproductive isolation, later generations of an isolated population will have a higher frequency of a mutation than the original population. For example, Ashkenazi Jews were segregated from the rest of the population and lived in separate communities for hundreds of years. Today, one percent of the Ashkenazi Jewish population--one person in 40--carries a 185delAG mutation in BRCA1, which places them at higher than the average risk for breast and ovarian cancer.

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