One final point that needs to be understood about BRCA1 and BRCA2 genes is a concept known as the "founder effect." The majority of people who have a BRCA1 or BRCA2 mutation have a unique mutation—one that is specific to them and their family.
To date, hundreds of unique mutations have been identified in both BRCA1 and BRCA2. However, there are a few exceptions. For instance, specific recurring mutations have been found in individuals of Ashkenazi Jewish descent, and persons from the Netherlands, Iceland, and Sweden. Mutations recur in these groups because of a founder's effect.
"Founders" are a small group of people who, due to geographic or religious isolation, interbred. When a small group of people interbreeds over generations, specific rare mutations can recur and become more common in the population. This is called a founder's effect.
The present day Ashkenazi Jewish population arose from a small group of founders, of whom one or more must have carried specific mutations in the BRCA1 and BRCA2 genes. In particular, there are three mutations (two in BRCA1 and one in BRCA2) that account for the majority of the BRCA mutations seen in persons of Ashkenazi Jewish ancestry. This information has practical meaning when it comes to genetic testing because some laboratories now offer "ethnic-specific" mutation panels. Rather than searching through the entire gene every time a person is tested, in some cases, laboratories can instead first look for specific mutations based on a person's ethnic background.
The founder effect is also important in Ashkenazi Jewish individuals because it has led to an increased occurrence of BRCA mutations in this population. In the general population, it is estimated that between one in 400 and one in 600 individuals has a mutation in BRCA1 or BRCA2. In contrast, one in 40 Ashkenazi individuals has one of the recurring mutations. This increased occurrence has implications in terms of assessing the significance of a family history of breast and ovarian cancer in Ashkenazi versus non-Ashkenazi persons.
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