The Gc groups of the plasma are the expression of genetically controlled variants of a protein belonging to the a-globulins, which is the carrier of vitamin D. There are numerous alleles of which only two, Gcl and Gc2, are relatively common. They are distinguished by methods based on electrophoresis. The distribution of the alleles and the possible relation of this to sunshine are described on p. 123.()
The protein concerned was known by its physical and immunological properties as a component of plasma, and it was known to have a number of hereditary variants characterized by different speeds of migration on electrophoresis. But for many years after all this had been discovered, it had no known function. Then it was found that the Gc proteins are the carriers of vitamin D. This vitamin either comes from food or is synthesized through the effect of the ultraviolet part of sunlight acting on a substance in the skin. Vitamin D is essential for the proper absorption of calcium from food, and so for proper bone formation, and a deficiency of it causes the bone disease known as rickets.
It is now known that the hereditary variants of the Gc proteins differ in their efficiency as vitamin D carriers, and it is to be expected that the more efficient one will prevail in conditions of poor sunlight. There are indications that this is the case, for Gc2 is on the whole more common, and Gcl less so, in conditions of poor sunlight. The relation is however not a constant one, and the problem is currently under intensive investigation.