The T-13910 allele is believed to impart to adults the ability to digest lactose. Those who possess it are able to drink milk and consume other dairy products without gastro-intestinal discomfort or the other problems associated with lactose intolerance.
I think this study
, Lactase persistence-related genetic variant: population substructure and health outcomes
, might have been mentioned here before. Its authors concluded, for one thing, that lactase persistence (the ability to digest lactose because one is still producing the enzyme lactase
) increases as one moves SE to NW in the British Isles. That struck me because I think it apparent that L21 also increases in frequency as one moves SE to NW in the British Isles.
Could there be a connection?
There is probably no way to prove that there is, but it is interesting to consider the possibility.
Here's the pertinent quote from the study's abstract:
We found an overall frequency of 0.253 for the C (lactase
non-persistence) allele, but with considerable gradients of decreasing
frequency from the south to the north and from the east to the west of
Britain for this allele.
Notice that the authors make the statement concerning the negative, that is, lactase non-persistence, rather than about lactase persistence. But if lactase non-persistence DECREASES as one one moves SE to NW in the British Isles, then it follows that lactase persistence INCREASES in the same direction.