05/08/2018
The White Spotting Series
Most white spotting on dogs is determined by the genes on the S locus . When we use the term "white spotting" we simply mean white areas on the dog, not actually white spots. White spotting can occur on any colour , and will cover up both eumelanin and phaeomelanin. In technical terms this is known as epistasis . So any dog can have white markings, whether they're black, blue, liver, isabella, brindle, sable, tan-pointed, merle or whatever.
White hair occurs when the skin cells are unable to produce any pigment . The white spotting gene impairs the ability of cells on particular parts of the skin to make pigment, so the skin becomes pink and the fur white. Nails and paw pads will also become pink in areas where pigment is not produced.
So far, only two white alleles have been proven to exist on the S locus:
S - no white
s p - piebald
A third allele may exist for "extreme white" (s w), however this has not been proven and so far all dogs with high white have been shown to be homozygous for s p instead.
The white spotting alleles are thought to be examples of incomplete dominance . This means that a heterozygous dog will express its most dominant gene, but may also be affected by the more recessive one to a lesser extent. For example an Ss p dog may have some white spotting (see below). However, the relationship between the alleles is complicated and can vary between breeds.
It has been shown that some dogs with white spotting do not have an s p allele at all. These are mostly dogs with "true" irish spotting (in other words, irish spotting that breeds true - this should be made clear further down the page). The allele that causes this pattern has not yet been identified and it is not known if it is also located on the S locus. For the purposes of this site we will refer to this gene as s i , but remember that it is most likely located on a different locus to s p . s i is much less common than s p and only occurs in certain breeds.
Spread of White
Whichever white pattern a dog has, its white will always follow the same rules of spread. White starts on the farthest "edges" of the dog - the tail tip, the tip of the muzzle, the paws and the tip of the breastbone. This is known as the "trim" pattern. From there it spreads to cover the muzzle and forehead, the front of the chest, the lower legs and more of the tailtip, creating irish spotting . Next it spreads round from the front to the back of the neck, and creeps up the legs and tail. On a piebald dog, only the head, back and tail base may still be coloured. The back colouring is the next to go, followed by the tail base, then the face markings. The ears will always remain coloured unless the dog has a very high amount of white. The ears are generally the last part of the dog to turn white.
Thks,
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