The story says the specific gene was found.
The evolutionary adaptation that produced reduced melanin to promote health in northern latitudes has long been known. The mutated gene causing that adaptation has now been specifically determined
Exactly. The discussion of the adaptation of a lower melanin content in northern latitudes (and southern latitudes in alternate low light continents) is to the correct point. There are several comments there that have it completely backwards. In lower light, there is less penetration to the inner skin layers where the chemical reaction of the conversion of cholesterol (NO!!) to the precursor compound to vitamin D, 7-dehydrocholestanol is caused by .....sunlight!!
The addition of vit D to milk has a history directly related to the amount of sunlight ANY person was able to get, black or white, in areas of great pollution or reduced sunlight. You have correctly explained that the gene coding for the melanin pigmentation selectivity has been identified. The gene has to be “triggered” by something, and that something is the amount of light, or in the case of high melanin production, too much light. The net result is that too much vit D KILLS a person. There is a lot about vit D (fat soluble) that is generally poorly understood. But a fast test for white folks is.. drink a half gallon of vit D supplemented milk and then go get a mild sunburn. You will have a massive vit D overdose headache very slow to get rid of without major aspirin use.