The high incidence of cystic fibrosis in white people seems to be something of a mystery. Suggestions have been made that carrier status conveys some selective advantage in the way that it does with sickle cell anaemia. What could this advantage be?
We know that the UV radiation in sunlight photolyses provitamin D3 into previtamin D3, which spontaneously isomerises to vitamin D3, which is in turn converted into the active hormone that plays an essential role in the control of calcium and phosphorus metabolism. Peoples living in regions of high radiation can therefore metabolise their own vitamin D3.
As peoples migrated northwards, sunlight decreased, particularly during the wintertime; and the further north they went the worse the situation became. Lack of vitamin D causes rickets and this could have been a major limiting factor in these migrations. The change to a white skin was obviously of selective advantage by letting ...
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