by Patrick Randall Breeders Education Committee
The basics of the color and color pattern inheritance in the Newfoundland have been known over 50 years. They were worked out by examining the outcomes of many breedings with many different breeds and deducing the number of loci and generally what they controlled, at least in terms of outward phenotype. These early results describing xx loci were largely from Little (1957) with a few other major contributors. Across all the breeds they looked at, it was a major intellectual achievement and it's remarkable how many things they got right.
We are relatively lucky with the Newfoundland. There are only three alleles to worry about accounting for the vast majority of color variation in the breed. It's important to identify the action of each locus: the B locus which determines whether black pigment can be formed in pigmented coat (rather than brown); the D locus which determines the intensity of pigment (either full intensity or dilute) and finally the S locus which determines whether the dog is solid ("self") colored or spotted. Note that these are entirely different actions and they are inherited completely independently. In fact, now that the locations of these genes are known to be on different chromosomes, we can say with a high degree of certainty that their inheritance is independent.
The content of this article is not necessary for a Newfoundland breeder or owner. It is, we think, interesting and reinforces the extent to which these loci are very separate entities and may prevent some confusion regarding color inheritance. For example, it should be clear by the end, that it is potentially misleading to say that black is dominant to grey-we should instead say that full intensity pigment is dominant to dilute pigment.
Pigment is formed by specialized cells called melancytes. Embryologically they originate in an area known as the "neural crest" (on the back close to the developing spinal cord) and during development migrate from there to their eventuallocation. So there is a developmental pattern in pigmentation in which everyone starts out as an extreme piebald (Sealyham terrier) goes through being a Landseer and eventually at the end of the migration which extends into post natal life, we end up, in the solid black dog, with these cells migrating across the entire body meeting at the chin and belly.
Some of these cells eventually take up residence in hair follicles. They extend long processes (called dendrites) and these dendrites can "inject" pigment in small packets called melansomes into the hair follicle as it grows. The melanocytes can synthesize two forms of pigment, eumelanin (dark pigment, brown or black) and pheomelanin (yellow pigment). Whether yellow or dark pigment is formed is under the control of both local and systemic hormones. If a membrane protein is stimulated by Melancortin Stimulating hormone, dark pigment is formed, if not (or if the action of the hormone is blocked, by say, the agouti protein) yellow pigment is formed. In the Newfoundland, outside of the rare black and tan (a rare recessive and the tan is pheomelanin, not brown), we only need worry about dark pigment, eumelanin. Once the syntheisis is directed toward dark pigment, and additional protein (Tyrosinase related protein I) is necessary for the formation of black, rather than brown eumelanin.
The three genes we are concerned with affect three different aspects of this process. The B locus is identical with the previously mentioned Trpl and the recessive brown allele does not allow the formation of black eumelanin, only brown. However, one good copy of the gene is enough to allow the formation of black throughout the coat so the brown is recessive.
The D, or dilute allele, affects the ability of the melanocyte to transport pigment down the long process of the melanocyte, so that it is not concentrated as efficiently. The protein coded by the D allele, melanophilin is one that helps hold onto the melanin on one side and another protein on the other as a little molecular machine makes its way along a fiber in the melanocyte dendrite toward it's eventual transfer to the hair shaft. This is very much like one of those horizontal cranes in a factory that carry heavy loads along a cable (or maybe a cable car). Interestingly, the dilute mutation is apparently in a regulatory site on the gene and not in an area that causes a defective protein to be made. It simply doesn't make enough. As with the B locus, one good allele is enough to produce full intensity pigmentation and dilute is recessive.
Finally, the Landseer allele at the S locus encodes a protein that helps support the melanocyte survival, MITF or microphthalmia-associated transcription factor. Like many proteins it gets its name from a disease associated with a mutation in the human. There are no known deleterious effects of the Landseer mutation at this locus in the dog. Note that there are multiple mutations of the gene, we're talking about one of them. Clearly, there are a lot of ways we could have affected the eventual distribution of melanocytes. For example, we could have changed or slowed the migration by making changes in chemical factors that help control it, we could have changed the tissue through which it was migrating. Anything we do would have the potential of creating a similar phenotype since it is the result of the normal developmental pattern. In fact the first thing scientists did was to rule out two genes (KIT and EDRN, mutations of which are known to cause white spotting in other species). It is also true that white spotting in general is not always as benign as it is in the Newfoundland. Since several related cell types originate in the neural crest and migrate from there, e.g., the autonomic nerve ganglia, the ear, disruptions in migration can have serious consequences.
It's clear then that these three genes are separate entities. "Black" is not dominant to Landseer. Solid colored is dominant to Landseer (and to Irish spotting which we are still not sure about at a molecular level). Black is not dominant to grey, but full pigmentation is domininant to dilute. Since the inheritance at these three loci is independent, it is clearly possible to breed for any combination of these three phenotypes, e.g., creme and white landseers (the creme being a dilute (D allele) brown).
In the past, the only way we had of determining the color genotype of dogs was to infer it from breedings to dogs of known genotypes (e.g., a known heterozygote or an "affected"). The last several years has seen the development of genetic testing for these mutations. A test for the D and B allele have been available for some time and the Landseer, or pied, allele is now coming on-line.
Resources:
A convenient summary of dog color genetics
reprinted from Newf Tide 1Q 2009