Well, putting it simply, populations belonging to specific races, ethnic groups, and other groups tend to have a particular distribution of genetic variation compared to the general population or contrasting groups. This is why having a particular background is a good reason to get tested for genes like BRCA1 (such as hailing from Ashkenazi Jewish descent). The risk of a particular individual with a BRCA mutation for developing a malignancy depends on multiple factors but increases overall. For example, having a biallelic mutation would pose the greatest risk, but other concurrent somatic mutations of course play a role. A child of a parent with a BRCA mutation would have a 50% chance of inheriting such a mutation. Therefore it is possible that overtime the progeny produced by a population with a higher BRCA1/2 frequency mixing with that of a lower frequency could have a lower risk.

It is known that the pattern of deposition (distribution) of fatty tissue in the female (and even male) body differs from one race to the next. Although I have personally not done research on breast cancer genes, it is easy to imagine that the more fat a woman (or man) has on the mammary glands, the more chance s/he has of developing breast cancer, simply because there are more nucleation sites for neoplastic tissue to start. That is my own take, from a non-expert on BRCA genes

BRCA1 mutation is a mutation that can be traced back to the XI century according to theoretical studies to central Europe jewish population. Probably to Russia. In those days, due to high endogamic behavior, the mutation remained mainly in Ashkenazy population. Ghettos in those days, contributed strongly to keep the mutation limited to this population.

Thanks all.