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.
I grew up drinking unpasteurized milk. There is quite a bit of difference in milk just in this last generation. Antibiotic "contamination", for one example. Another would be the breed of milk cow. Dairy farms are much rarer now. When you see one, you'll likely see "Holsteins". {colored like a Dalmatian} Not long ago, you would see Jerseys, Guernseys, Milking Shorthorns, and maybe others. The difference in the milk was very noticeable, even though the lactose couldn't be that much different.
I'm fascinated by some old recorded accounts from the Ozarks of an illness called "milk sick". They believed it was caused by the cow eating some type of plant and contaminating the milk. These old doctors could diagnose this condition from the odor when entering the home.
Most interesting.
The darwinists are getting about as desperate as the researchers throwing millions of dollars at the futile search for the "gay gene". It's a terrible thing with science gets politicized.