Nowadays all Shelties have white markings. The “si” gene is responsible for this phenomenon. This “si”, which applies to Irish pattern, is recessive to “S” that gives a solid color coat. The “si” makes the pigmentation impossible on certain parts on the body and white is left in it's place. Some Shelties have the recessive white factor “sp” instead of the “si”. Geneticist named this “sp” the piebald gene. When a dog is homozygous for the “sp” we see a coat with a lot of white of over the body and sometimes only the dogs head is colored. They gave this dog the name color-headed white. Don't confuse this color-headed white with the double merle white dogs. These color-headed white Shetland Sheepdogs are healty dogs but according to the English standaard patches of white on the body are highly undesirable. Sometimes a white factored Sheltie (“si sp”) has a white spot on his body, but in most cases you can't see the difference between a “si si” and “si sp”, although some have a white stripe up to the knee. There appear to be modifying factors involved on the S series. The majority of plus modifiers will result in more pigment (less white) while the minus modifiers enlarge the areas of white. This explains why some Shelties have huge shawl collars and others have a small splash. So a dog who is homozygous for the Irish pattern (“si si”) and who has a majority of minus modifiers can have the same amount of white as a white factored dog (“si sp”) who carries a large number of plus modifiers.
