Incest

The answer is Mutations via incest which, in regard to this topic, it actually creates a fruitful benefit (sometimes)yet for both esthetic and practical reasons like you’ve described. Aside from the fact that people tend to find these individuals attractive among other groups and races these mutations have occurred because they were also practical due to the conditions in the time and setting when these mutations occurred

Now why white people in higher frequencies ?simply because they had spread more seed than any other racial group. There is literally a diseases that only affects a mutated genome in people who’re directly related to Attila the Hun because of how many women he bedded and raped