Lactose tolerance is a mutation. Lactose intolerance is the original form of the gene.
Scientists have looked into the human genome to find the precise genetic mutation that causes lactose tolerance. Lactose tolerance in humans always come from the same genetic mutation, regardless of whether they are of European, Asian, and African descent.
Most people in the world are lactose intolerant. People do not naturally have the ability to digest milk beyond infancy. The sugar, lactose, that is present in high levels in milk causes nausea and bloating in most adults. Europeans and Americans are much more likely to have the genetic mutation that allow them to digest lactose. Lactose intolerance is present in only 5% of Northern Europeans but nearly 100% of Southeast Asians.