Lactose intolerance is not a disorder.
It is actually more common in the
world than lactose tolerance.
Originally, people in the
world were all lactose intolerant. Their bodies ceased the production of the enzyme lactase after an early
age.
Their diets did not contain much
milk anyways, so it was not a problem.
Somewhere in Europe, a mutation occured where lactase continued to be produced through adulthood.
Lactose tolerance is a dominant allele, so the trait became very common in Western Europe.
Person 1:
Wow, You're lactose intolerant? Is that some kind of
freak mutation?
Person 2:
Actually, you have the mutation, sir.