Hey, It's a crevo thread. Someone has to throw the first grenade.
Let's say it's a recessive, and the other one is dominant.
You could have a population of people able to drink milk no matter what, but if the mother were carrying one of each, and had at least two kids, and one kid inherited the dominant gene, and the other kid inherited the recessive gene (the father having had neither gene), one kid could drink milk and other couldn't.
Right in the same family.
So the mad scientist comes along and discovers this family has three lactose handling genes. One dominant ~ to drink. One recessive - to drink. One "old fashioned", and to drink as a child then shut down.
Which, by the way, makes sense for a friend of mine who has several children. Some of them grew up drinking milk into adulthood. Others did so only as children. He could never drink milk, nor could his wife.
The only answer we could come up with was that he and his wife both had at least one recessive for drinking milk but everything we could find suggested the milk drinking gene was dominant.
From this one case I'm going to hazard a guess that the new gene found in Africa is a recessive.
Looks like the two different genetic strains can still cross-breed too! No species difference has yet appeared.