Certain populations worldwide are much more lactose intolerant than others. Why might this be the case?

Lactose persistence, and therefore lactose tolerance, is inherited as a dominant trait. Lactose intolerance is the result of  having two identical alleles of a particular gene or genes. for the recessive lactase allele that is poorly expressed after early childhood.

You basically can get it from your parents.

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answer; polycythemia is the correct answer;