One of the most common reasons, perhaps the most common reason why mismarks will appear in breeds is that countless color genes in the canine world are recessive in some way. This makes it quite easy to forget about a gene that was present in the early days of a breed, but disappeared through selective breeding. It only takes one breeding for a recessive to disappear, and if chance favors the production of homozygous dominant individuals and heterozygous carriers, that gene may soon be forgotten. Also, only breeding from the dominant phenotype will lead to production of some homozygous dominant individuals, and eventually a population may end up being mostly made up of homozgous dominant and heterozygous individuals. Of course, since heteozygotes are still in the population, the production of the recessive "mismark" is still very possible. Depending on the percentage of heterozygotes, it may even be almost guaranteed. Remember that if you breed heterozygotes together, you have a 50% chance of producing more heterozygotes (as well as 25% homozygous dominant and 25% homozygous recessive). Even if you take the "undesirable" recessive phenotypes out of the breeding pool, two-thirds of the remaining individuals will still carry the recessive. The likelihood that the recessive will disappear completely is basically zero.
In addition, if there is some sort of recessive being carried by members of a breed, inbreeding will often lead to it appearing at a higher frequency than would be expected if matings occur between more distantly related individuals (see Why Inbreeding Leads to Increased Homozygosity). Remember: all members of a purebred population are relatives due to the small number of individuals that went into starting basically every breed. Often, purebreds have very high inbreeding coefficients, sometimes to the point that, no matter how little relationship two dogs have in their recent pedigrees, they may in fact be as genetically similar as cousins or even closer relatives. So, the more heavy the inbreeding (i.e. the higher the inbreeding coefficient), the more likely that a recessively inherited mismark will appear. If breeders wish to avoid the colors that are seen as "bad," then this is one more reason to add onto the giant pile of reasons to not inbreed.
|This silver Labrador is expressing the recessive blue dilution gene (possibly in combination with the liver gene). It is believed by most Labrador breeders out there that the presence of the gene in the breed is the result of a fairly recent out-cross to a Weimaraner. If this is true, the silver Labs will likely be no healthier thanks to the out-cross when compared to regular Labs due to the heavy inbreeding involved in creating continuous lines of silvers. In fact, their health may even be worse than the rest of the breed thanks to that same inbreeding.|
|These are two white (albino) Doberman pinschers, which are caused by an incomplete form of albinism (making them cream and white with green-yellow eyes). The sort of heavy inbreeding situation I've mentioned happened when the white Doberman appeared on the scene, which is why white Dobermans are far more prone to health and behavioral issues than the rest of the Doberman breed. Sensitive skin is basically guaranteed due to the lack of pigment.|
Needless to say, I think anyone who is interested in any "unusual" color in a breed should be very wary. I would say that virtually every breeder that breeds specifically for a color that is not accepted by a breed standard should be approached with extreme care. If the color is recessive, then it is likely that they are involved in very questionable breeding practices. Part of this is that the lack of acceptance from other breeders toward the "unacceptable" color has likely resulted in a very narrow pool of dogs going into creating the lines that they are offering. I reiterate: the narrower the gene pool, the worse off a population will be. If any breeder brags about having a particular color of puppy available, this should be a major warning sign. If that puppy is being sold for a higher price than the "normal" puppies, this is major warning sign number two.
One issue I see with modern genetic research is the possibility that all members of a population may be tested for a particular color gene in an attempt to completely eliminate it. To me, this idea is completely absurd, but I wouldn't put it past some breeders. It should be much more important to deal with inherited health issues rather than genes controlling color, coat type, and other traits in a breed.