By Dr. Leanne Bertani, Adrienne Wilder, & JP Yousha
Last year, the Japanese Chin Club of America Health Committee embarked on a study of coat color. We collected cheek swab samples from Japanese Chin of various colors. The samples were sent to Dr. Sheila Schmutz of the University of Saskatchewan for DNA testing at the A, B, E, and K loci. We are finally ready to share what we learned!
Background & Terminology
In order to understand the study results, it may be helpful to review a bit of basic terminology. We describe dogs by genotype and by phenotype. Genotype is the inherited information in the genes; phenotype denotes the observable characteristics.
Locus is simply the location of a gene; the place it lives on a particular chromosome – its street address if you will. Loci is the plural. An allele is a form of a gene at any given locus. At a locus that determines color, each dog has two alleles, one from each parent. If the two copies of the allele are the same, the dog is said to be homozygous for the allele. If the alleles are different from each other, the dog is heterozygous at that location. A dominant allele may change the dog’s color when the dog has just one copy, while a recessive allele generally requires two copies to definitively change a trait. A dominant allele is depicted by a capital letter; a recessive allele is depicted by a lower case letter.
Melanocytes are the pigment-producing cells of the body. Where there are no melanocytes, there can be no color, so the absence of the melanocytes causes white spotting. Pigment in dogs comes in two basic forms. Eumelanin is the dark pigment that is black, brown, or (in other breeds) blue. Phaeomelanin is the bright pigment that produces red, yellow, and cream colors in canine coats. Each type of pigment is affected by different genes. Some genes function to affect the intensity of the pigment; other genes are “pattern” genes that may alter the distribution of the pigment. Genes that produce white coats actually disable the body’s ability to produce pigment in the hair.
Many of us learned canine color genetics from the old but classic book by Clarence Little, The Inheritance of Coat Color in Dogs (Howell Book House 1957). While Little was remarkably accurate in many of his theories, modern molecular studies have demonstrated that there are more or fewer alleles at some genes than originally postulated.
Let us look at some of the genes that affect coat color in the Japanese Chin.
The S locus – Spotting (miTF)
Clarence Little suggested that there were four alleles of the Spotting (S) Locus, but recent research suggests that there may be several genes involved in spotting, each with two or three alleles. One gene that causes a spotting pattern in some breeds was identified last year as MITF (microphthalmia associated transcription factor), a critical gene in the pigmentation pathway; but there is still much to be learned about the genes involved in white spotting.
We did not test any of our study subjects for spotting genes, so for now, we can only say that the white areas on the dog represent an absence of melanocytes and therefore, an absence of color. These areas may vary in size and location, but the Japanese Chin breed is usually white on the chest, lower legs, and tip of the tail. Breeders tend to select for a white muzzle, a white blaze between the eyes, and patches of color surrounding the eyes and ears. The preference for these color patches on the head may be partially protective against a type of deafness that occurs in piebald dogs. Patches are distributed somewhat randomly elsewhere on the body, with the color of these patches largely determined by the loci A, B, E, and K.