A breed of pig often bred and raised by traditional methods, resulting in meat with good flavour and fat-to-lean ratio.
All pigs (and indeed all mammals, including us) have a gene (called MC1R) which is expressed in skin pigment cells called melanocytes and determines the colour of skin and hair.
Wild Boars have 2 copies of the "normal" MC1R and are brownish. If a pig has 2 copies of a mutant inactive MC1R it is pinky-red, like the Tamworth. If a pig has one normal and one OVERactive copy of the MC1R it is black, like the large black.
The Old Spot is the result of a really odd combination of mutations in the MC1R.
It has two mutations in the same MC1R gene. One of them is a superactive one, the other is a so-called frame shift, which, if present in a cell, wins out over the overactive mutation because it stops any gene product being made so the over active mutation never gets a chance to be seen. However the frameshift mutation is a bit unstable and it can spontaneously disappear during the pigs development, exposing the overactive form of the gene in those cells where the frameshift, through random chance, has disappeared. This explains not only the black spotting but also the fact that no two Gloucester Old Spots are alike in their pigmentation as the frameshift disappears at random during development. (With thanks to Steve O'Rahilly.).