[GourmetDan]

"However, certain human populations have undergone a mutation on chromosome 2 which results in a bypass of the common shutdown in lactase production, allowing members of these populations to continue consumption of fresh milk and other milk products throughout their lives."

http://www.reference.com/browse/wiki/Lactose_intolerance

[AndrewC]

I understand that, but lactose digestion is normal(what I am saying). It is "shut down" later in life as a "normal" condition of some mammals including man(what you are saying). Lactose tolerance is "returned" by not "shutting down" the mechanism that metabolizes the sugar. If you look at the genome on gene 2 you might see something related to your answer. Again note that turning something on or off requires a mutation(change) to the mechanism coded in DNA.

LAC, 2q21

Also HYPOLACTASIA, ADULT TYPE has a good explanation of the topic