rs4988235: G or A
AA or AG – can drink milk
GG – lactose intolerant

Most mammals including people cannot drink milk as an adult.  This is because the gene gets shut off later in life.

The ability to drink milk requires the enzyme lactase. People who can still drink milk as an adult have a DNA change in a region near the gene for lactase (LCT) that keeps the enzyme on into adulthood.  People who can’t drink milk as an adult are called lactose intolerant but since this is really how most people are, scientists have taken to calling people who can drink milk as an adult as having lactase persistence. 

This particular variant was selected in parts of Europe after the domestication of cattle.  Others exist in other parts of the world.

More info on lactose intolerance genetics at The Tech Museum.