Used with permission from Today's Breeder, Nestlé Purina PetCare Company.
Breeders have long been interested in coat color. If they could predict coat color, they could make breeding decisions much easier. Through techniques such as DNA testing and gene mapping, scientists are learning insightful information about coat color in dogs.
Nearly 50 years ago in 1957, Clarence Little wrote "The Inheritance of Coat Color in Dogs." His text, which was written before DNA testing, became a classic on coat color. Working with breeding records he obtained from breeders and from his own crossbreedings of dogs of different coat colors, Little developed a comprehensive hypothesis to explain most of the colors and patterns in dogs.
For the past few years Sheila Schmutz, Ph.D., professor of animal and poultry science at the University of Saskatchewan, and her colleagues have studied genes involved in the pigmentation pathway at the DNA level. "Many of Little's predictions have held true but some have not," she says. "DNA research has shown that there are more genes involved in coat color than those hypothesized by Little and that the actual number of alleles at genes he discussed is more for some genes and fewer for other genes."
Schmutz's group has found DNA polymorphisms, or markers, in several genes in the pigmentation pathway. Using these markers to map specific genes to specific
chromosomes, they have been able to follow family studies to determine if the same variant of the marker is always inherited with a particular coat
color, pattern or associated health problem.
"We've learned that multiple mutations in one gene (TYRP1) cause brown color with none having an ill effect on health," she says. "Yet in other coat color genes there may be multiple effects, including some on health. For example, the gene that causes merle when homozygous usually results in deafness and sometimes can also result in serious eye problems. We studied several families of Australian Shepherds and excluded several genes as the cause of merle."
Currently the Canadian scientists have expanded their study to include Harlequin and merle Great Danes using the polymorphisms in these same genes. Results
thus far are more optimistic, Schmutz say "Harlequin Grat Danes are thought to be heterozygous at both the Halequin and merle-causing genes and so
finding which gene causes the Harlequin pattern may also lead us to find the gene that causes merle."
In her own kennel of Large Munsterlanders, Schmutz had a litter of dogs with a coat color disorder called black follicular dysplasia in which areas of the coat that are normally black are gray at birth and later the weak hairs break and fall out. The underlying darkly pigmented skin is also wrinkled and sometimes pimply. "We tested the litter and successfully showed that several genes do not cause the condition," Schmutz says. "Still it's a mystery what does cause the condition. Colleagues at the University of Pennsylvania and collaborators in Germany are going to continue studies of this disorder."
This purebred Kerry bitch whelped one wheaten-colored Kerry (a male, shown here at approximately 1 year old) in an otherwise normally marked litter
Schmutz's group recently identified the mutation in the MC1R (melanocortin receptor 1 gene) gene that may cause melanistic mask, which is helpful for Dr. Sheila Schmutz breeders of dogs in which masking is important. "In a few breeds of dogs, a black mask appears to be a fixed trait," Schmutz says. "But, that's not the case with Great Danes, where masking is a variable trait that, when present, is visible on dogs with fawn and brindle coats. The dominant mask trait can also be carried by black or blue Great Danes and harlequins and merles; it's just not noticeable because of their dark coloring or spotting pattern."
Within the past few months, DNA testing has become available for the masking trait. Now a breeder can test a black-masked fawn to determine if it is homozygous for black mask and thereby know in advance that all the puppies the dog will have, irrelevant of the sire chosen, will also have a black mask. Alternatively, if the bitch tests heterozygous for mask, a breeder may choose to search out a sire that is homogenous for black mask.
Dog breeders can contribute to genetic research on coat color as well as other traits, Schmutz says." If breeders keep careful records of traits in their parents and puppies, these can he used to determine how traits are inherited."
More information on color genetics: http://www.dogfencediy.com/library/color-genetics/