Please if anyone could help me with a few suggestions about how to answer this question.

Question

A mutation produces a change to produce the dominant gene for continued lactase production throughout adult life. This occurs very frequently and is present in almost 100% of people in northern Europe and populations orginating from northern Europe now living else where. In people originating in most parts of the world (as well as other mammals) the recessive gene for non-persistence of lactase is most common. The dominant gene is thought to be a fairly recent mutation, orginating perhaps 6000-10 000 years ago in people developing the skills of cattle herding and dairy farming.

Suggest how this gene may have increased in Northern European populations but not in other global areas? I would be grateful for some help really really stuck on how to answer this one.

This article may help you.

http://en.wikipedia.org/wiki/Lactose_intolerance#History_of_genetic_prevalence

Thank you