This silver colouring is produced by a gene which is called Dilute. This dilute gene is a recessive trait and a Silver Labrador dog would have to have inherited a copy of this dilute gene from both parents. The dilute gene in black Labradors produces Charcoal puppies. In Yellow Labradors the puppies would be Champagne coloured (not visible but dogs have a grey nose!) and the Chocolate Labradors bred with the dilute gene produce these Silver Puppies. The recent explosion in the Chocolate Labrador population has substantially increased the chances for silver Labradors to appear naturally.
This dilute gene is also found in many other dog breeds and the silver colouring which results is accepted by the Kennel Club in Chows, Chesapekes, Newfoundlands, Weimaraners, Whippets, and many other breeds.
The known origins of the Labrador retriever started out in the areas of Newfoundland and Chesapeke, where it started life as a St Johns Water dog. As both Newfoundland and Chesapeke Retrievers can posess this dilute gene, it is likely that it has been in the genetics of some Labradors all along!