seems like a reasonable starting point.
It should be noted that what you've communicated about your conversation with your doctor appears to be a rather muddled version of the facts:
- there is no single "defective gene for breast cancer." Breast cancer is a number of different diseases which have a multitude of different causes, and only a small minority of breast cancer cases are attributable to genetics. Your doctor was probably referring to tumors that result from having a bad copy of the BRCA1/BRCA2 tumor suppressor genes, which are indeed attributable to genetics and are disproportionately prevalent in Ashkenazi Jews, but there are other genes (p53 and several others) which also have been found to cause breast cancer.
- Ashkenazi Jews are nowhere close to 10% of all global breast cancer diagnoses. There are about 2.1 million breast cancer diagnoses per year globally, and I am pretty confident that there are not 210,000 diagnoses per year among the 5 million or so Ashkenazi women in the world. Maybe they are 10% of all diagnoses among the small subset of breast cancer diagnoses associated with BRCA1/BRCA2 mutations, although I am inclined to doubt this also, and I'm too lazy to try and hunt down the exact numbers right now. I suspect that your doctor might have been referring to a different statistic with the "10%" figure: Out of all the breast cancer diagnoses per year among Ashkenazi Jewish women, about 10% of those are attributable to BRCA1/BRCA2 mutations.
- BRCA1/BRCA2 mutations were not inherited by all Ashkenazi Jews. Only about 1 in 40 Jewish women have a mutation in one of these genes.
- Ashkenazi Jews are not the only population with BRCA1/BRCA2 mutations, but the reason for this is not "some non-Jewish person a thousand years ago was the first human being on Earth to have the mutation, and then they or one of their descendants converted to Judaism, resulting in both Jews and non-Jews being susceptible." Rather, there were dozens of different (though still rare) mutation events affecting the same BRCA1/BRCA2 genes that occurred at various times to people of a whole bunch of different ethnicities, and it's simply that for a variety of reasons (chiefly, that Ashkenazi Jews were a small and historically insular population), the single mutation that occurred in an Ashkenazi person centuries ago has been unfortunately conserved among Jews to a degree that other mutations to the same genes have generally not been when they occurred in people of other ethnicities. There is a chart of all the different identified mutations to BRCA1/BRCA2 that have been associated with people of dozens of different ethnicities: Dutch, Japanese, American Indians, etc. at https://en.wikipedia.org/wiki/BRCA1