So how exactly does the diet of Mongolians prove lactase persistence is not fundamentally genetic? Lactose intolerance isn't something like celiacs where a dash of the substance causes adverse reactions. Most of Asia still sell non-lactose-free dairy in supermarkets. Look up any Youtube video of Korean street foods and you will see dairy products such as processed cheese, yogurt, mil, and ice cream are ubiquitous in Korea. That doesn't detract from the fact that the vast majority of the population are lactose intolerant in the definition that they no longer produce enzymes that can break down lactose once reaching adulthood.