A small note, in addition to the other response provided
to your question. You might want to look into some literature
by William Durham, "Coevolution:genes,culture, and human diversity".
He provides a great explanation and overview of this phenomena.
Specifically, he shows that lactose intolerance is associated with
vitamin D availibiliy(found in cows milk, among other things). In
cultures where the vitamin D could be gained through sunlight, then
lactose intolerance is high, conversely, as you go up in latitude, so
does the ammount of sunlight(uv-radiation) decrease. Therefore,
there is a trend for european cultures to be more lactose tolerant,
since their survival depends on it.